Question: A unit of time or time unit is any particular time interval, used as a standard way of measuring or expressing duration, the exact modern definition, is from the National Institute of Standards and Technology (NIST), a measurement standards laboratory, and a non-regulatory agency of which organization?

Context paragraphs:
National Institute of Standards and Technology: The National Institute of Standards and Technology (NIST) is a measurement standards laboratory, and a non-regulatory agency of the United States Department of Commerce.  Its mission is to promote innovation and industrial competitiveness.

Instrument error: Instrument error refers to the combined accuracy and precision of a measuring instrument, or the difference between the actual value and the value indicated by the instrument (error).  Measuring instruments are usually calibrated on some regular frequency against a standard.  The most rigorous standard is one maintained by a standards organization such as NIST in the United States, or the ISO in European countries.  However, in physics—precision, accuracy, and error are computed based upon the instrument and the measurement data.  Precision is to 1/2 of the granularity of the instrument's measurement capability.  Precision is limited to the number of significant digits of measuring capability of the coarsest instrument or constant in a sequence of measurements and computations.  Error is ± the granularity of the instrument's measurement capability.  Error magnitudes are also added together when making multiple measurements for calculating a certain quantity.  When making a calculation from a measurement to a specific number of significant digits, rounding (if needed) must be done properly.  Accuracy might be determined by making multiple measurements of the same thing with the same instrument, and then calculating the result with a certain

NIST-F1: NIST-F1 is a cesium fountain clock, a type of atomic clock, in the National Institute of Standards and Technology (NIST) in Boulder, Colorado, and serves as the United States' primary time and frequency standard.  The clock took less than four years to test and build, and was developed by Steve Jefferts and Dawn Meekhof of the Time and Frequency Division of NIST's Physical Measurement Laboratory.

National Physical Laboratory of India: The National Physical Laboratory of India, situated in New Delhi, is the measurement standards laboratory of India.  It maintains standards of SI units in India and calibrates the national standards of weights and measures.

Korea Research Institute of Standards and Science: The Korea Research Institute of Standards and Science (KRISS) is the national measurement standards laboratory for the Republic of Korea.  It is a government-funded institute responsible for providing national measurement standards and advancing measurement technologies.  KRISS is also an active member of the General Conference on Weights and Measures (CGPM).

Measurement microphone calibration: In order to take a scientific measurement with a microphone, its precise sensitivity must be known (in volts per pascal).  Since this may change over the lifetime of the device, it is necessary to regularly calibrate measurement microphones.  This service is offered by some microphone manufacturers and by independent testing laboratories.  Microphone calibration by certified laboratories should ultimately be traceable to primary standards a (National) Measurement Institute that is a signatory to International Laboratory Accreditation Cooperation.  These could include the National Physical Laboratory in the UK, PTB in Germany, NIST in the USA and the National Measurement Institute, Australia, where the reciprocity calibration (see below) is the internationally recognised means of realising the primary standard.  Laboratory standard microphones calibrated using this method are used in-turn to calibrate other microphones using comparison calibration techniques (‘secondary calibration’), referencing the output of the ‘test’ microphone against that of the reference laboratory standard microphone.

Time point: In music a time point or timepoint (point in time) is "an instant, analogous to a geometrical point in space".  Because it has no duration, it literally cannot be heard, but it may be used to represent "the point of initiation of a single pitch, the repetition of a pitch, or a pitch simultaneity", therefore the beginning of a sound, rather than its duration.  It may also designate the release of a note or the point within a note at which something changes (such as dynamic level).  Other terms often used in music theory and analysis are attack point and starting point.  Milton Babbitt calls the distance from one time point, attack, or starting point to the next a time-point interval, independent of the durations of the sounding notes which may be either shorter than the time-point interval (resulting in a silence before the next time point), or longer (resulting in overlapping notes).  Charles Wuorinen shortens this expression to just time interval.  Other writers use the terms attack interval, or (translating the German ""Einsatzabstand""), interval of entry, interval of entrance, or starting interval.

NIST Special Publication 800-53: NIST Special Publication 800-53 provides a catalog of security controls for all U.S. federal information systems except those related to national security.  It is published by the National Institute of Standards and Technology, which is a non-regulatory agency of the United States Department of Commerce.  NIST develops and issues standards, guidelines, and other publications to assist federal agencies in implementing the Federal Information Security Management Act of 2002 (FISMA) and to help with managing cost effective programs to protect their information and information systems.

Accumulator (energy): An accumulator is an energy storage device: a device which accepts energy, stores energy, and releases energy as needed.  Some accumulators accept energy at a low rate (low power) over a long time interval and deliver the energy at a high rate (high power) over a short time interval.  Some accumulators accept energy at a high rate over a short time interval and deliver the energy at a low rate over longer time interval.  Some accumulators typically accept and release energy at comparable rates.  Various devices can store thermal energy, mechanical energy, and electrical energy.  Energy is usually accepted and delivered in the same form.  Some devices store a different form of energy than what they receive and deliver performing energy conversion on the way in and on the way out.

Unit of time: A unit of time or time unit is any particular time interval, used as a standard way of measuring or expressing duration.  The base unit of time in the International System of Units (SI), and by extension most of the Western world, is the second, defined as about 9 billion oscillations of the caesium atom.  The exact modern definition, from the National Institute of Standards and Technology is:

Supporting paragraphs: Unit of time National Institute of Standards and Technology
Answer: United States Department of Commerce