"My pastor tells us regularly that we need to be tithing our income. I'm don't object to supporting the church, but when he teaches about it he quotes passages that say we should tithe, but never any passages that say what the tithe is. What is a tithe?"
The word tithe comes from the word meaning tenth and was used in the Old Testament to indicate the portion of one's income that was set apart for God. In modern church usage it has come to mean the standard unit of systematic giving of one's income to the church. Most churches plead for their congregants to pledge and to give a tenth of their gross income to the work of the church, a pleading that comes with a good bit of biblical support.
The tithe itself has endured a radical evolution from the time of its inception to the present. The historical basis for the tithe seems to come from Abraham and Jacob. In Genesis 14 Abraham gives Melchizedek, the king of Salem, a tenth of all he owns. The writer of Hebrews promotes this king to high priest, thus justifying the offering Abraham gave. However, it seems more likely this sharing was offered by way of tribute to insure the safety of the wandering family. However, Jacob makes the first promise of a tenth to God when he sets up the altar at Bethel in Genesis 28.
Jacob's promise of a tenth seems to have been a votive offering, or part of fulfilling a vow. However, when the earliest law came to be written, the tenth became the tithe and it became a requirement for all households. The tithe, in its earliest form, was a randomly chosen tenth. In Leviticus 27 every tenth animal through the gate was selected, whether blemished or not, and given as the tithe. If by chance the tenth animal through the gate happened to be the prized animal then the owner could "redeem" the animal for cost plus 20%. However, what was to be done with the tithe, i.e., who got it, isn't commanded in Leviticus, it just states that it belongs "to the Lord" (Lev. 27:30).
There are two different commands concerning what the tithe is to be used for. The command issued in Numbers 18 best represents what the church has adopted. Here the tithe is taken to the sanctuary for the direct support of the priest. The priest and his family could dispose of the tithe as they saw fit, consuming where they pleased (many offerings had to be eaten conspicuously in the sanctuary). The priest who receives the tithe was required to tithe himself from the received portion. Also, the Numbers passage required that the tithe was no longer just a random tenth, but it was to be the best of the produce.
However, problems arise when you turn to the commandments on tithing in Deuteronomy. Most Bible scholars posit that this book was compiled during the reign of Josiah when the Temple in Jerusalem became the central focus of the nation. During Josiah's reign the sanctuaries, where worship had once been authorized, were generally banned and the nation was required to make pilgrimages to Jerusalem to make their offerings, pay their tithes, and worship. To make this an attractive requirement it seems that the use of the tithe was changed. In Deuteronomy 12 and 14 the tithe was only given to the priests every third year. The other two years the tithe was taken to the Temple and then consumed by those making the offering (Deut. 14:22-23). Indeed, if one lived too far from the Temple to bring their tithe in satisfactorily, they were allowed to exchange their produce for money, make the trip to Jerusalem and then: "spend the money for whatever you wish--oxen, sheep, wine, strong drink, or whatever you desire. And you shall eat there in the presence of the Lord your God, you and your household rejoicing together" (Deut. 14:26).
The use of the tithe has changed much since then. Today the tithe is used for the general support of the church, its ministers, and its programs. For those who find comfort and solace in the church the tithe seems to be a minimal gift for such services. But note, the history of the tithe cannot be used to browbeat or intimidate one into giving--unless of course the minister is overly selective in defining the historical tithe.