By default, autocompletion search will include Term names as well as Synonyms. The other options to choose from include:
Term name - The name of the term itself.
Synonym - The name of the synonym for a term.
Definition - Partial-matches are based on inclusion within the definition of a term.
Obsoletes - This is an optional setting to be used in combination with one of the settings above. It will look for matches to term name, synonym, or definition within those terms that are obsolete, in addition to non-obsoleted terms. In order to select obsolete search, you must have at least one of the other three selected.
To change these settings, simply check on/off the desired box in the Settings menu: