הלכה על נדה 70:5

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.