Catalyzes the removal of sialic acid (N-acetylneuraminic acid) moieties from glycoproteins, oligosaccharides and gangliosides. Hydrolysis of alpha-(2->3)-, alpha-(2->6)-, alpha-(2->8)- glycosidic linkages of terminal sialic acid residues in oligosaccharides, glycoproteins, glycolipids, colominic acid and synthetic substrates. Belongs to the glycosyl hydrolase 33 family.