One's 'thing'--(1) A point of personal interest; a hobby, sport, or avocation that succinctly defines a person. (2) A brief coupling of words used to evoke someone's personality in a small-talk setting: (3) A laconic summation of one's character and interests used for the purpose of categorization and judgement. See also 'What do you do?
Every one has experienced how learning an appropriate name for what was dim and vague cleared up and crystallized the whole matter. Some meaning seems distinct almost within reach, but is elusive; it refuses to condense into definite form; the attaching of a word somehow (just how, it is almost impossible to say) puts limits around the meaning, draws it out from the void, makes it stand out as an entity on its own account.