Thessaloniki vs Salonica

Thessaloniki and Salonica both are nouns.

Word	Noun	Adjective	Verb	Adverb
Thessaloniki	Yes	No	No	No
Salonica	Yes	No	No	No

As nouns, Thessaloniki and Salonica are synonyms defined as:

Thessaloniki and Salonica: a port city in northeastern Greece on an inlet of the Aegean Sea; second largest city of Greece

Other synonyms of Thessaloniki include Salonika, Thessalonica.

Thessaloniki (noun)	Salonica (noun)
a port city in northeastern Greece on an inlet of the Aegean Sea; second largest city of Greece	a port city in northeastern Greece on an inlet of the Aegean Sea; second largest city of Greece