It depends on who you ask, a botantist or horticulturist. For all practical purposes I say its a fruit. It's legal and technical mumbo-jumbo that claims it to be a vegetable.
Botanically speaking, the tomato is a fruit and is classified as a berry. It is a fruit because it is a ripened mature ovary containing seed. One can confirm this by looking up both "fruit" and "tomato" in a dictionary. (No not the definition of old slang that says a "tomato" is an attractive woman). Things like peppers, eggplants and cucumbers are also fruits by this definition.
Horticulturally speaking it is defined as a vegetable for two other reasons:
1- it is a nonwoody annual (ok, what I want to know then why dont they say watermelons are vegetables?)
2- it was classified this way in 1893 by the Supreme Court for trade reasons (imposing import taxes to protect growers)
One can find out more and verify this info from the following site: