Entry 2 senses · 2 variants Webster, 1913 Tau /(tou)/ · IPA /taʊ/ n. n. 01 n. The nineteenth letter (Τ, τ) of the Greek alphabet, equivalent to English t. 1. The nineteenth letter (Τ, τ) of the Greek alphabet, equivalent to English t. 02 n. The common American toadfish; -- so called from a marking resembling the Greek letter tau (τ). 1. The common American toadfish; -- so called from a marking resembling the Greek letter tau (τ).(Zool.) Phrases & compounds Tau cross — See Illust. 6, of Cross.