Definition of 'Time Series'
A sequence of numerical data points in successive order, usually occurring in uniform intervals. In plain English, a time series is simply a sequence of numbers collected at regular intervals over a period of time.
Investopedia explains 'Time Series'
Time series analysis can be useful to see how a given asset, security or economic variable changes over time or how it changes compared to other variables over the same time period. For example, suppose you wanted to analyze a time series of daily closing stock prices for a given stock over a period of one year. You would obtain a list of all the closing prices for the stock over each day for the past year and list them in chronological order. This would be a one-year, daily closing price time series for the stock.
Delving a bit deeper, you might be interested to know if a given stock's time series shows any seasonality, meaning it goes through peaks and valleys at regular times each year. Or you might want to know how a stock's share price changes as an economic variable, such as the unemployment rate, changes.