What is the meaning of ς?

The lower case letter sigma of the Ancient Greek alphabet when used as the final letter in a word (a final sigma). In all other positions in a word the normal sigma (σ (s)) is used.

The lower case letter sigma (σίγμα τελικό (sígma telikó)) of the Greek alphabet when used as the final letter in a word (a final sigma). In all other positions in a word the normal sigma (σ (s)) must be used.

Source: wiktionary.org