In morphology (the linguistics kind), productivity is a potential property of word formation rules, namely, the rule being able to be used 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 that has never encountered that word - they may smile, but they will understand what I mean. I may 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.