Did Ferdinand de Saussure define meaning in his Course in General Linguistics?
I don't recall whether he explicitly defined meaning in a generalized form in the Course, but if you just want to know how he's considered to have defined it, then I can tell you Saussure referred to it as the "signified," and treated it as the impression in the mind resultant of a signifier. In other words, meaning is a mental construct which refers to the signifier (for example, a physical object). Together (and, to Saussure, inseparably), the signifier and the signified constitute a sign. However, he cautioned that "it is a great mistake to consider a sign as nothing more than the combination of [signifier] and [signified]," as the signified is as continuous in nature as its signifier; to quote Fredric Jameson, "our understanding proceeds from one whole or Gestalt to the other, rather than on a one-to-one basis."
In the simplest sense, Saussure considered meaning to be the psychological experience of the impression of that toward which it refers (limited neither in multiplicity nor by corporeality). In more modern terms, you could think of meaning as an encoded form of an observation, the encoding of which is a function of the entirety of local semiotic reality (i.e. how you know anything is related to everything you know; how an observation is represented is related to the sum of representations thereto. Descartes noted this thusly: "...the diversity of our opinions, consequently, does not arise from some being endowed with a larger share of reason than others, but solely from this, that we conduct our thoughts along different ways, and do not fix our attention on the same objects."). Hope that helps!