Is talking to yourself a medical condition?

my dad talks to himself all day or whenever hes not asked anything from real people. hes 68 and is very critical and angry with the world and expresses this when he is talking to himself. the trouble is he is very vocal and doesn't seem to notice when people are watching, for instance when my kids are asking him something or when my partner walked past his bedroom door and notice him threatening his make believe friend or foe. if you snap him out of it he gets quite aggressive and snappy especially with my mum. he lives in his imaginary world more than the real world. has he lost it? he wont see anybody about it as i think a) he does not know hes always doing it and b) the embarrment. im just curious if it is medical. i know he wont go to docs. its a shame as he used to do when i was younger but not as much im 32 now and he hardly talks to anybody else but himself. we have had some family crisis with losing two kids to cot death and two kids in care has he never got over that?