A time series is an ordered sequence of observations. Although the ordering is usually through time, particularly in terms of some equally spaced time intervals, the ordering may also be taken through other dimensions, such as space.
Time series occur in a variety of fields.
In agriculture, we observe annual crop production and prices. In business and economics, we observe daily closing stock prices, weekly interest rates, monthly price indices, quarterly sales, and yearly earnings. In engineering, we observe sound, electric signals, and voltage. In geophysics, we record turbulence, such as ocean waves and earth noise in an area. In meteorology, we observe hourly wind speeds, daily temperature, and annual rainfall.