let ‧ ter ‧ box ‧ ing /ˈletəˌbɒksɪŋ $ -terˌbɑːks-/ BrE AmE noun [uncountable]
when a film from the cinema is broadcast on television with black bands at the top and bottom of the television screen so that the film will have the same ↑ dimension s as it did on a cinema screen