A set of timestamped data entries. Each time series consists of an identifier (such as stock ticker ACME), and multiple timestamp-value pairs (such as all trading days and the closing price for ACME on each trading day).
Sequential data collected in the field with a regular occurance interval and roughly 20 or more repititions. For example, hourly groundwater conductivity (SpC) data collected for a day or more, daily precipitation data collected for three weeks or more, or monthly lake stage data collected for two years or more.
Observations of a variable made over time. Time series can also be constructed from a cross-sectional study if the same questions are asked more than once over time. See also longitudinal study.