Tosefta for Niddah 8:2

And what is the difference between a zov and an emission of seed (Lev. 19:20, "שכבת זרע")? Zov comes from "dead flesh" (i.e., a flaccid penis, see Niddah 35b:5, Steinsaltz tr.), and an emission of seed comes from "live flesh" (i.e., an erect penis). Zov is runny, and similar to the white of an unfertilized egg. And an emission of seed is viscous, and is similar in appearance to the white of an egg that is not unfertilized. An emission of seed is reddish and the woman may attribute [a bloodstain on her clothes] to it (see Niddah 8:2).