A term that refers to two different things: an option position and a difference between the prices.
If we are talking spread options, it is the position which is established by simultaneous purchase and sale of two options of the same class but different series (different strike prices or/and expiration dates). The spread itself is the difference between strike price and the market value.
If we are talking price spreads, it is the difference between the bid and the ask price of security or an asset, influenced by standard price determination factors (supply vs. demand, total number of shares or assets available etc.).