For questions about sequential data indexed by time. Time intervals between data may be regular or irregular.

A time series is a series of data points indexed (or listed or graphed) in time order, so it is a sequence of discrete-time data. There are generally 3 distinguishing features of time-series data:

  1. Arriving data is usually recorded as a new data point (appended to the data).
  2. Data arrival is usually ordered by time.
  3. The primary axis is time though the time intervals between data points may be regular or irregular.

Time series can be used to form predictions and have substantial applications in economics and signal processing.

For further details see Wikipedia: Time Series.