The term “closed Sicilian” can refer to a variety of anti-Sicilian variations where White avoids “d4”. However, the term “Closed Sicilian” is actually the name of a common form of closed anti-sicilian, where White plays Nc3 and a Kingside fianchetto with g3 and Bg2 (and later “f4”). The main line is almost symmetrical with Black also doing a Kingside fianchetto.
Diagram: Closed Sicilian (Main Line)
Other anti-Sicilian openings that are also called types of “closed Sicilians” include: