The term 'deep in thought' is used to describe a person who is LOST in their own thoughts, often for an extended period of time. This can be expressed in a variety of ways, such as cogitating (10 letters), RAPT (4 letters), LOST (4 letters), PENSIVE (7 letters), PREOCCUPIED (11 letters), and thinking (8 letters). All of these words imply that the person is so focused on their thoughts that they are unaware of their surroundings.