Startup is the time period between the invocation of an action and its active time frame.
The startup is a state in which the command has been recognized, and the wind-up for the move has begun. Startup includes the invincibility preceding the appearance of the attack hitbox or opening caused by lack thereof, the gauge stock expenditure before a super flash, etc.
In a narrower context, the term may be used to herald the instant of appearance of the hitbox of interest. Afterwards come the active frames.