See Definitions :
John Locke distinguished, in his Essay, “real essence” from “nominal essence.” Nominal essence, according to Locke, is the “abstract Idea to which the Name is annexed (III.vi.2).” Thus, the nominal essence of the name ‘gold’, Locke said, “is that complex Idea the word Gold stands for, let it be, for instance, a Body yellow, of a certain weight, malleable, fusible, and fixed.” In contrast, the real essence of gold is “the constitution of the insensible parts of that Body, on which those Qualities [mentioned in the nominal essence] and all other Properties of Gold depend (III.vi.2).”
A rough way of marking the distinction between real and nominal definitions is to say, following Locke, that the former states real essence, while the latter states nominal essence. The chemist aims at real definition, whereas the lexicographer aims at nominal definition.
This characterization of the distinction is rough because a zoologist’s definition of “tiger” should count as a real definition, even though it may fail to provide “the constitution of the insensible parts” of the tiger. Moreover, an account of the meaning of a word should count as a nominal definition, even though it may not take the Lockean form of setting out “the abstract idea to which the name is annexed.”
Perhaps it is helpful to indicate the distinction between real and nominal definitions thus: to discover the real definition of a term X one needs to investigate the thing or things denoted by X; to discover the nominal definition, one needs to investigate the meaning and use of X.
Whether the search for an answer to the Socratic question “What is virtue?” is a search for real definition or one for nominal definition depends upon one’s conception of this particular philosophical activity. When we pursue the Socratic question, are we trying to gain a clearer view of our uses of the word ‘virtue’, or are we trying to give an account of an ideal that is to some extent independent of these uses? Under the former conception, we are aiming at a nominal definition; under the latter, at a real definition.
Your purported "eight-legged invertebrate" definition pf "tiger" is false due to zoologist’s (real) definition of “tiger” that is currently "agreed" by the scientific community as the correct one.