1. (a.) Done or occurring in an instant, or without any perceptible duration of time\; as, the passage of electricity appears to be instantaneous.

2. (a.) At or during a given instant; as, instantaneous acceleration, velocity, etc.

alert apt brief compact compendious concise curt curtal curtate decisive decurtate expeditious fast hair-trigger immediate instant lightning-like little low momentaneous momentary presto prompt punctual quick quick as lightning quick as thought rapid ready short short and sweet speedy spontaneous succinct summary swift synoptic transient transitory