what's tt
â¢ TT (noun) The noun TT has 3 senses: 1. (astronomy) a measure of time defined by Earth's orbital motion; terrestrial time is mean solar time corrected for the irregularities of the Earth's motions. 2. a republic in the western central Pacific Ocean in association with the United States. 3. a country scattered over Micronesia with a constitutional government in free association with the United States; achieved independence in 1986. Familiarity information: TT used as a noun is uncommon.