A denomination is a large, mainstream religious organization, but it does not claim to be official or state sponsored. It is one religion among many. For example, Baptist, African Methodist Episcopal, Catholic, and Seventh-day Adventist are all Christian denominations.
The term ecclesia, originally referring to a political assembly of citizens in ancient Athens, Greece, now refers to a congregation. In sociology, the term is used to refer to a religious group that most all members of a society belong to. It is considered a nationally recognized, or official, religion that holds a religious monopoly and is closely allied with state and secular powers. The United States does not have an ecclesia by this standard; in fact, this is the type of religious organization that many of the first colonists came to America to escape.