Noun
prosign (plural prosigns)
English Wikipedia has an article on:prosignWikipedia
Any of a set of special sequences in Morse code used as control characters and punctuation.
Each Morse code symbol represents either a text character (letter or numeral) or a prosign and is represented by a unique sequence of dots and dashes. Source: Internet
In the United States the final commercial Morse code transmission was on July 12, 1999, signing off with Samuel Morse's original 1844 message, "What hath God wrought", and the prosign " SK ". Source: Internet
For those Q-code assertions that merely need to be acknowledged as understood, the usual practice is to respond with the prosign or which means "understood". Source: Internet