The protagonist is the central figure of a narrative work, the subject around which the plot develops.
The term was born in ancient Greece, to indicate the first actor of a theater company. Similar figures are the deuteragonist (the second most important character, sometimes a guest star), the co-star (where there are more central figures for the purposes of the plot) and the antagonist (rival or enemy).
In literature the protagonist is, generally, the hero of a story told from his own point of view: however, he is not necessarily the narrator, as the roles of the protagonist and the narrator may not fit into any pre-established scheme.
Being the most popular character, his evolution is crucial in contributing to the interest and rhythm of the work.
A well-known narrative device concerns the "false protagonist", that is a character initially emphasized but then overshadowed by the appearance of another.
The co-star indicates an actor who plays one of the main roles in a play, film or television or in the case of the co-stars by each of the actors who play equally important roles in a film.
Wiktionary contains the dictionary lemma "protagonist"
Protagonist, on dictionaries.corriere.it, Corriere della Sera.
Protagonist, in Treccani.it - Treccani Vocabulary online, Institute of the Italian Encyclopedia.
πρωταγωνιστής, Henry George Liddell, Robert Scott, A Greek-English Lexicon, in Perseus Digital Library and Online Etymology Dictionary