Thessaloniki vs Thessalonica

Thessaloniki and Thessalonica both are nouns.

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

As nouns, Thessaloniki and Thessalonica are synonyms defined as:

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

Other synonyms of Thessaloniki include Salonica, Salonika.

Thessaloniki (noun)	Thessalonica (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