noun
Refers to a person from the same country. Can also be used for someone from the same province or region within a country, though less common.
海外で同国人に会うとほっとする。
I feel relieved when I meet a fellow countryman abroad.
noun
person from said country
Used when the country is already clear from context, often in contrast to other nationalities.
この店は同国人が経営している。
This shop is run by a person from that country.
同郷人 specifically means a person from the same hometown or region, whereas 同国人 can refer to the same country or province.
Compound of 同 (same) + 国 (country) + 人 (person).