The term 'BRIEF INSTANT' can be broken down into two words, 'BRIEF' and 'INSTANT'. BRIEF is an adjective meaning short or concise, while INSTANT is a noun meaning a very short period of time. The crossword solutions for this term are SEC (3 letters) and TRICE (5 letters). SEC is an abbreviation for second, which is a unit of time equal to one sixtieth of a minute. TRICE is an archaic term meaning a very BRIEF period of time, usually less than a second. Therefore, the term 'BRIEF INSTANT' can be defined as a very short period of time, usually less than a second.