Is definition a word in the Scrabble Dictionary

Yes, definition is playable

...and is worth 16 points.

Found in the following dictionaries

Enable (Words with friends)
SOWPODS (Collins Scrabble Words)
OTCWL2014 (NASPA Word List 2014)
TWL06 (NASPA List 2006)

"Definition" Definition

noun

A statement conveying fundamental character.
A statement of the meaning of a word, phrase, or term, as in a dictionary entry.
The act or process of stating a precise meaning or significance; formulation of a meaning.
The clarity of detail in an optically produced image, such as a photograph, effected by a combination of resolution and contrast.
The degree of clarity with which a televised image or broadcast signal is received.