Sifrei Mitzvot sur Le Lévitique 15:3

That a man, having a running issue, defiles (ibid. 15:1-15).

The commandment of the matter of impurity of a zav (one with a discharge) to be impure and to render impure: That a zav be impure and render others impure, as it is stated (Leviticus 15:2-3), “when there be any man with a discharge from his flesh, etc.” And the explanation comes, “from his flesh,” is meaning a flow from the orifice of the member. And [it is] as they, may their memory be blessed, expounded, “‘When there be a discharge’ — it is possible, a discharge from any place would be impure. [Hence,] we learn to say, ‘from his flesh’ and not all of his flesh. Once Scripture has distinguished between flesh and flesh, I have merited to learn out the law of impurity with a zav and with a zavah. Just like a zavah is from the place that she becomes impure, etc.”

And the content of discharge is that which issues as a type of fluid from a man from the orifice of the member and he has no desire or pleasure from its emission. And they, may their memory be blessed, said (Niddah 35b) that it is similar to barley dough and, likewise, similar to the white of an addled egg; but semen is not like this, since it is similar to the white of an egg that is not addled. And that is the difference between them to distinguish between this and that.