Definition - What does Horizontal Spread mean?
Testopedia explains Horizontal Spread
A horzonntal spread is also known calendar spread or time spread and contains one long option and one short option of the same class with different expiration months. It is called a horizontal spread because of how the options are listed in the option chain or in the newspaper. An investor who is long a horizontal spread would purchase the option with the longer expiration and sell the option with the shorter expiration. This would result in a net debit in the investor's account.