What is Calendar Spread?

Legal Definition
In finance, a calendar spread (also called a time spread or horizontal spread) is a spread trade involving the simultaneous purchase of futures or options expiring on a particular date and the sale of the same instrument expiring on another date. The legs of the spread vary only in expiration date; they are based on the same underlying market and strike price.

The usual case involves the purchase of futures or options expiring in a more distant month and the sale of futures or options in a more nearby month.
-- Wikipedia
Legal Definition
In finance, a calendar spread (also called a time spread or horizontal spread) is a spread trade involving the simultaneous purchase of futures or options expiring in a particular month and the sale of the same instrument expiring in another month.