Explicit definitions of words or terms are put in quotation marks.
Such definitions may or may not be direct quotations from a dictionary or similar source.
Definitions that follow such expressions as means, defines, or is defined as are normally put in quotation marks. This highlights or emphasizes the definition.
Definitions that follow the verb to be normally are not put in quotation marks since such definitions are seen as the same as a predicate nominative..
Examples:A kiltie is a fringed leather flap found on some shoes.
(The verb to be is used.)
Kiltie means "a fringed leather flap found on some shoes."
Kiltie is defined as "a fringed leather flap found on some shoes."