The ripened seed-bearing part of a plant when fleshy and edible. In other words, a fruit is any fleshy material covering a seed or seeds. Most fruits, from a horticultural (science of cultivating) perspective, are grown on a woody plant, with the exception of strawberries.

Or you can say, generally a fruit is the edible part of the plant that contains the seeds. So your eggplant, tomato, cucumber and zucchini are fruits.