You should always quote definitions as the source of a definition can provide telling information about how you are using the term being defined. For instance, if you are using a dictionary definition, your reader might assume you are using the term more generally. If you are using a definition from a textbook or article, the definition might be more focused and have a specific contextual meaning, representing the bias of the author.

As a tip, the preferred dictionary of academic writing is the Oxford English Dictionary. Using this, as opposed to dictionary.com or other dictionaries, can boost your ethos as a scholar!