The Greek word logos is variously translated: "explanation," "definition," "characterizartion," "account."

a logos of X must:

(a) be applicable to all instances of X (not be too narrow: a list, such as Meno (and Euthypho) give, would always be too wide unless exhaustive)

(b) not be applicable to things that are not X (not be too wide)

(c) not contain in the defiens (that which does the defining) any mention of the defiendum (what is to be defined): see Meno 79b-c.

For example, Socrates gives the following logos of "shape" (Meno 75c):

"That which alone of existing things always follows [i.e., accompanies] color."

an acceptable account (logos) must:

identify the criterion (or form) whereby all the many things called X are rightly called X.

This could be a characteristic, or property, common to all the members of a set or class, in virtue of which they belong to the class.

For example,

if ``pleasure-producing'' is the correct logos of virtuous acts, then each and every virtuous act must produce pleasure, and all acts that are virtuous must be virtuous because they produce pleasure.

See also Plato's account of knowledge.