The definitio quid rei, that is, the real definition, is a definition of a term, so that it is indicated what the thing, as it is in extramental reality, and as it is signified by the term, essentially is (that is, what it is per se). This is done by way of subsumption into some class. The term is the intellectual counterpart of the word and so also the meaning of that word. The term (as natural sign) stands for the thing and already embodies what the thing is, but (still) unanalyzed. The nominal definition gives the meaning of the word, by pointing to the object ( = ostensive definition) or by enumerating diagnostic features (which often are not essential). After having pointed to the object or to the diagnostic features (because ultimately something must be pointed to) the natural sign is formed (intensio animae) through which the word refers to the thing. The nominal definition of, for example, a phoenix is a description by means of terms, which, each for themselves, do have a significatum in re (that is, which do refer to entities in extramental reality). However, the combination of these terms does not refer to something in extramental reality. Indeed the nominal definition of the term ' phoenix ' reads : a bird that re-emerges again from its ashes.
The nominal definition leads from a word ( = conventional sign) to a natural sign (concept), but this concept has, in the case of a phoenix, not a significatum in re.
The nominal definition is in fact the answer to the question "what does one say, as regards what the given thing is?" This in the following sense : how one usually characterizes it. And now things either cannot go beyond the presented nominal definition (as in the case of the phoenix), or things can be followed further, ending up at the real definition, and thus a genuine definition.