In morphology (the linguistics kind), productivity is the property of word formation rules that the rule can be applied to create new words, not listed in any dictionary or previously encountered by the speaker.

An example: the suffix -ish, used to create adjectives out of adjectives.
I can talk about something greatish before an audience which has never encountered that word - they may smile, but they will understand what I mean by it. I can even encounter an adjective I've never seen before in my life and instantly apply -ish to it to form a new adjective, and its meaning will be clear to whoever understands the meaning of the original adjective.