A denomination—as defined by Webster—is “a class or kind with a specific name or value.” Coins (pennies, nickels, dimes, quarters, half-dollars, dollars) are different denominations of the class of “money.” They are all money, but they are different denominations of money.
In the context of the Bible and Christianity, a denomination is an organization that is Christian that has a number of local churches under its auspices. In order to qualify as a denomination, the organization must believe that salvation comes through faith in Jesus Christ and have some sort of authority over more than one local church. (Some denominations only have two or three churches.) These organizations would be denominations of Christianity. Organizations like the Jehovah Witnesses, Roman Catholicism, the Seventh Day Adventists, and the Mormons, would not be considered denominations of Christianity because they don't believe that salvation comes through faith in Jesus Christ alone. Organizations like the Baptists, the Lutherans, the Methodists, the Presbyterians, the Assemblies of God organization, the Foursquare Gospel organization, would be considered denominations of Christianity because they all believe that salvation comes through faith in Jesus Christ. Some of these denominations actually have several sub-categories, each of which would be a denomination (e.g. American Baptist, Southern Baptist, Evangelical Lutheran, Missouri Synod Lutheran).
Denominations are NOT of God. Denominations are the work of man and are contrary to the Word of God. The Bible says that God calls some men to the office of pastor and then places them in the office when they are ready. There is to be NO ONE between the man of God that God called to the ministry, and God. The pastor was called by God, the pastor was placed in the ministry by God, and the pastor receives all his instructions from God. The instructions as to what the pastor is supposed to do in his ministry come directly from God. They do not come through anybody else. In a denominational structure, the pastor is somewhat accountable to his denomination. He is not free to just hear from God and obey God.
Some churches join various organizations or associations or fellowships. If the organization or association or fellowship that the church joins has any say-so about how the individual church operates or what they preach and teach, etc., the organization or association or fellowship is a denomination and the church has become denominational. It doesn't matter that the organization or association or fellowship says that they are not a denomination. If they have any say-so or authority over the churches, they are a denomination.
Some churches, therefore, are members of more than one denomination.
Some churches have “branch churches.” These churches with “branch churches” would also be denominations. It doesn't matter that the pastor who is over the “main church” and also over these “branch churches” doesn't consider it a denomination. It is a denomination none-the-less. Each church is supposed to have its own pastor who is accountable only to God.