When a person smokes enough marijuana to feel its effects, but not enough to alter competence at regular daily tasks or create a lasting low after the effects have worn off.
Becoming functionally high is a skill that comes naturally for some, and for others takes a long time to develop and master.
Person 1: I just got some killer bud from a friend of mine. Wanna smoke?
Person 2: Ok, but I have work in an hour. I guess I'll go in functionally high.