Left over and leftover are two version of the same term, which means remaining or not yet used as an adjective. Left over comes after verbs in sentences. Leftover comes before the noun it modifies. Leftover can also be a noun, where it refers to food that has not been eaten.

The one-word, unhyphenated leftover is usually an adjective preceding the noun it modifies (e.g., leftover pasta). When it follows the noun it modifies, left over is usually two words (e.g., there’s some pasta left over). Leftover also works as a noun, usually pluralized (e.g., save the leftovers).

Leftover, on the other hand, is an adjective that means the same thing, but you turn the phrase into a compound word when placing it before the remaining object. So the three dinner rolls that were left over can also be referred to as three leftover dinner rolls.

"Leftover" and "left over" are both adjectives and have the same meaning. What's the difference? "Leftover" can only be used before a noun.