A sequel, in games,books, and media, means that it is a story based on the events of a prior peiece of work in a direct fashion. The number of them is irrelevant. For 2, the original and one sequel, its called a Duology. The original with 2 sequels is called a Trilogy. An original with 4 sequels is a Quintet, and so on.
Examples:
KotOR 1 and 2: Duology.
X-Com 1, 2, and 3: Trilogy.
The Cleric Canticle books 1 through 5: Quintet.