A denomination, in the Christian sense of the word, is an identifiable religious body or grouping under a common name, structure, and/or doctrine drawn from the three major divisions of Churches: Roman Catholic, Eastern Orthodox, and Protestant.
The term may be applied in particular to Protestant church bodies or groupings. Notable among these are:
Within each of these groups there are sub-groups which are also called denominations. For example, within Baptists are the Southern Baptist Convention, American Baptists USA, and other smaller groups.
Congregations which have chosen not to identify with a particular denomination (though identifying with a particular group) are considered non-denominational. The majority of these congregations are Protestant. There are some groups which fall within a larger grouping but are non-denominational, an example is the Independent Baptist movement which falls under the overall Baptist group but who's congregations are not organized into a sub-group denomination.
The degree to which denominations differ and their acceptance within Christianity varies widely.
To speak of Denominationalism may presume that some or all Christian groups are, in some sense, versions of the same basic thing regardless of their distinguishing labels, as branches of the same identical tree, grown from the same root, producing the same kind of fruit. Not all churches agree to this; for instance, the Roman Catholic and Orthodox Churches do not include themselves under the term "Denominations", as the implication of interchangeability in the term "denominationalism" does not agree with their theological teachings.
There are some groups which practically all others would view as apostate and therefore not legitimate versions or branches of Christianity (John 15:1-6). Others may be seen as heretical, even by some churches of their own denomination. To be a heretic means only that one holds an erroneous doctrine of importance, not that one has renounced (apostatized from) the Christian faith.
A denomination can be organized in one of two ways:
Some denominations have both types of arrangements. For example, the Assemblies of God exist as a congregational structure at the individual level, but ministers are licensed and ordained on a hierarchial structure (i.e. at AG headquarters) and individual congregations must hire an AG licensed/ordained minister.