Oxford:

a recognized autonomous branch of the Christian Church.

Merriam-Webster:

a religious organization whose congregations are united in their adherence to its beliefs and practices

Cambridge (who literally use Catholicism as an example of a denomination in their example sentence):

a religious group that has slightly different beliefs from other groups that share the same religion:

Collins:

A particular denomination is a particular religious group which has slightly different beliefs from other groups within the same faith.

Oh, and from the Wikipedia article you cited above:

The term refers to the various Christian denominations (for example, Eastern Orthodox, Roman Catholic, and the many varieties of Protestantism)

I don't think it works to be a stickler for a prescriptivist definition of a word when that definition doesn't even match up with how dictionaries define urm