On December 16, 1773 a group of men protested unjust taxes by having a party in Boston. In emulation of this event, Americans across the land have been protesting big government and unjust taxes by having parties. In honor of the original, the moniker tea party is used by the participants in such a protest.
A tea party is an event.
But many, following the lead of the media, have taken to using the term Tea Party to describe the phenomenon as if it were a political party. In some cases this is wishful thinking by the leftist media. They would love to see a 3rd party which would split the vote of those opposing the socialists now in power. But mostly it is just carelessness or ignorance.
The tea party movement is neither a political party, nor a subdivision of a political party. It is rather a protest by citizens of the republic who have had enough of big government, corruption, hypocrisy, looting, and unjust taxation.