Impromptu and Off the top of one's head

Impromptu adverb - Without advance preparation.

Off the top of one's head and impromptu are semantically related. in off the cuff topic. In some cases you can use "Off the top of one's head" instead an adverb "Impromptu". informal substitute

Impromptu and off the top of one's head are semantically related. Sometimes you can replace term "Impromptu" with "Off the top of one's head", this terms are similar.