A fruit is an angiosperm structure developing from a flower's ovary, sometimes together with additional floral tissues. Its enclosing wall, the pericarp, develops from the ovary wall; a seed develops from an ovule. A fruit and a seed therefore represent different structures, even when they are closely attached.
A tomato is a fruit in this anatomical sense regardless of its culinary classification. Fertilization usually initiates coordinated seed and fruit development, but parthenocarpy produces fruits without fertilization. Tomato experiments manipulating auxin and gibberellin signaling separated the control of fruit initiation from the presence of developing seeds, illustrating why a fruit need not contain mature seeds.[ref]
The Interaction between DELLA and ARF/IAA Mediates Crosstalk between Gibberellin and Auxin Signaling to Control Fruit Initiation in Tomato by Jianhong Hu and others. The Plant Cell 30, pp. 1710–1728; genetic analysis of hormone signaling and parthenocarpy
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