official name of a country
Refers to the formal, legally recognized name of a nation, as opposed to common or abbreviated names.
The official name of Japan is 'Nihon-koku' (State of Japan).
Please write the official country name on the form.
国名 simply means 'country name' and can be informal or abbreviated; 正式国名 specifically refers to the official, formal name.
通称 is a common or popular name, often contrasted with the official name (正式国名).
Compound of 正式 (official, formal) + 国名 (country name).