Although there is a lot of debate over whether or not tomatoes are considered a citrus fruit, they are most definitely not. There are certain defining characteristics which citrus fruits have, such as the fact that they all grow in perennial trees, which tomatoes do not.
Tomatoes grow for the most part on shorter vine plants. While they may be technically considered to be perennial plants, tomatoes are usually grown as annuals, which differs greatly from citrus fruits. Some examples of citrus fruits include lemons, oranges, and grapefruits.
Tomatoes are part of the nightshade family which also includes eggplants and potatoes.