Webster’s Revised Unabridged Dictionary · 1913
Echo
Ech′o, n.; pl. Echoes (#). [L. echo, Gr. ἠχώ echo.] (Whist) (a) A signal, played in the same manner as a trump signal, made by a player who holds four or more trumps (or as played by some exactly three trumps) and whose partner has led trumps or signaled for trumps. (b) A signal showing the number held of a plain suit when a high card in that suit is led by one's partner.