adj. 1) referring to a person who is not able to manage his/her affairs due to mental deficiency (low I.Q., deterioration, illness or psychosis) or sometimes physical disability. Being incompetent can be the basis for appointment of a guardian or conservator (after a hearing in which the party who may be found to be incompetent has been interviewed by a court investigator and is present and/or represented by an attorney) to handle his/her person and/or affairs (often called "estate"). 2) in criminal law, the inability to understand the nature of a trial. In these cases the defendant is usually institutionalized until such time as he/she regains sanity and can be tried. 3) a generalized reference to evidence which cannot be introduced because it violates various rules against being allowed, particularly because it has no bearing on the case. It may be irrelevant (not sufficiently significant) or immaterial (does not matter to the issues).