The briefest of memories that occurs immediately following the reception of a stimulus?

According to Baddely & Hitch's multi-store model of memory, the store you are most likely referring to is the sensory store, also known as sensory memory. It features a relatively unlimited storage capacity, and a very brief duration of retention. Upon reception of a sensory stimulus, it will first be sent here before shortly either, if paid conscious attention to, being transferred to the short-term store, or lost.

Different types of sensory information will be stored for varying durations. For example, visual stimuli (iconic memory) will be retained for approximately 0.2-0.4 seconds, and auditory stimuli (echoic memory) will be stored for around 2-4 seconds.