Title: Key (music)
Content:  and thus end in a different key. This is an example of modulation. In rock and popular music some pieces change back and forth, or modulate, between two keys. Examples of this include Fleetwood Mac's Dreams and The Rolling Stones' Under My Thumb. "This phenomenon occurs when a feature that allows multiple interpretations of key (usually a diatonic set as pitch source) is accompanied by other, more precise evidence in support of each possible interpretation (such as the use of one note as the root of the initiating harmony and persistent use of another note as pitch of melodic resolution and root of the final harmony of each phrase)."
Title: Key (music)
Content:  In music theory, the key of a piece is the group of pitches, or scale, that forms the basis of a music composition in classical, Western art, and Western pop music. The group features a tonic note and its corresponding chords, also called a tonic or tonic chord, which provides a subjective sense of arrival and rest, and also has a unique relationship to the other pitches of the same group, their corresponding chords, and pitches and chords outside the group. Notes and chords other than the tonic in a piece create varying degrees of tension, resolved when the tonic note or chord returns. The key may be in the major or minor mode, though musicians assume major when this is not specified, e.g., "This piece is in C" implies that the key of the song is C major. Popular songs are usually in a key, and so is classical music during the common practice period, around 1650–1900. Longer pieces in the classical repertoire may have sections in contrasting keys.
Title: Modulation (music)
Content:  In certain classical music forms, a modulation can have structural significance. In sonata form, for example, a modulation separates the first subject from the second subject. Frequent changes of key characterize the development section of sonatas. Moving to the subdominant is a standard practice in the trio section of a march in a major key, while a minor march will typically move to the relative major. Changes of key may also represent changes in mood. In many genres of music, moving from a lower key to a higher often indicates an increase in energy. Change of key is not possible in the full chromatic or the twelve tone technique, as the modulatory space is completely filled; i.e., if every pitch is equal and ubiquitous there is nowhere else to go. Thus other differentiating methods are used, most importantly ordering and permutation. However, certain pitch formations may be used as a "tonic" or home area.
Title: Key (music)
Content:  the original key. In the Baroque it was common to repeat an entire phrase of music, called a ritornello, in each key once it was established. In Classical sonata form, the second key was typically marked with a contrasting theme. Another key may be treated as a temporary tonic, called tonicization. In common practice period compositions, and most of the Western popular music of the 20th century, pieces always begin and end in the same key, even if (as in some Romantic-era music) the key is deliberately left ambiguous at first. Some arrangements of popular songs, however, modulate sometime during the song (often in a repeat of the final 
Title: Key (music)
Content:  Methods that establish the key for a particular piece can be complicated to explain and vary over music history. However, the chords most often used in a piece in a particular key are those that contain the notes in the corresponding scale, and conventional progressions of these chords, particularly cadences, orient the listener around the tonic. The key signature is not always a reliable guide to the key of a written piece. It does not discriminate between a major key and its relative minor; the piece may modulate to a different key; if the modulation is brief, it may not involve a change of key signature, being indicated instead with accidentals. Occasionally, a piece in a mode such as Mixolydian or Dorian is written with a major or minor key signature appropriate to the tonic, and accidentals 
Title: Key signature
Content:  Key signatures are a notational device in diatonic or tonal music that define the key and its diatonic scale without the need for accidentals. Music can be notated using other means, and the key of a piece of music may not always conform to the notated key signature. This is particularly true in pre-Baroque music, when the concept of key had not yet evolved to its present state. More extensive pieces often change key (modulate) during contrasting sections. These sections are sometimes indicated with a change of key signature, but are sometimes indicated using accidentals.  The Toccata and Fugue in D minor, BWV 538 by Bach has a key signature with no sharps or flats, indicating that it may be in D, in the Dorian, but the Bs that occur in the piece are written with accidentals, making the music actually in D minor.
Title: Music
Content:  determines the scale used, which centres around the "home note" or tonic of the key. Simple classical pieces and many pop and traditional music songs are written so that all the music is in a single key. More complex Classical, pop, and traditional music songs and pieces may have two keys (and in some cases three or more keys). Classical music from the Romantic era (written from about 1820–1900) often contains multiple keys, as does jazz, especially Bebop jazz from the 1940s, in which the key or "home note" of a song may change every four bars or even every two bars.
Title: Key (music)
Content:   The key usually identifies the tonic note and/or chord: the note and/or major or minor triad that represents the final point of rest for a piece, or the focal point of a section. Though the key of a piece may be named in the title (e.g., Symphony in C major), or inferred from the key signature, the establishment of key is brought about via functional harmony, a sequence of chords leading to one or more cadences, and/or melodic motion (such as movement from the leading-tone to the tonic). For example, the key of G includes the following pitches: G, A, B, C, D, E, and F; and its corresponding tonic 
Title: Theoretical key
Content:  While a piece of Western music generally has a home key, a passage within it may modulate to another key, which is usually closely related to the home key (in the Baroque and early Classical eras), that is, close to the original around the circle of fifths. When the key is near the top of the circle (a key signature of zero or few accidentals), the notation of both keys is straightforward. But if the home key is near the bottom of the circle (a key signature of many accidentals), and particularly if the new key is on the opposite side (in the late Classical and Romantic eras), it becomes necessary to consider enharmonic equivalence (if double accidentals are to be avoided). In each of the bottom three places on the circle of fifths the two enharmonic equivalents can be notated entirely with single accidentals and so do not classify as 'theoretical keys':
Title: Musical syntax
Content:  The highest level of pitch organization can be seen in key structure. In Western European tonal music the key is based on a scale with its associated chords and chord relations. Scales can be built up as minor or major scales (differing in the succession of intervals between the scale tones) on each of the 12 pitch classes and therefore there are 24 possible keys in tonal music. Analysing key structure in context of musical syntax means to examine the relationship between keys in a piece of music. Usually, not only one key is used to build up a composition, but also so-called key "modulations" (in other words the alteration of keys) are utilized. In these modulations a certain recurring pattern can be perceived. Switches from one key to another 

Question: if a piece of music is perceived to have changed key then we say the piece has?
Answer: transposed.
transposition.