In the world of plants, a tomato is a fruit. When it comes to plants, fruits are the fully developed ovaries of blooming plants that hold seeds.
Tomatoes are botanically considered fruits because they have seeds and grow from the fertilised ovary of a tomato flower.
Though, when talking about food and daily language, tomatoes are often called vegetables.
This grouping is based on how they are usually used in cooking, not on how they are classified botanically.
Tomatoes are usually used in savoury dishes like salads, soups, and sauces. This fits with the culinary meaning of vegetables as plant parts that are eaten in savoury dishes.
So, even though a tomato is botanically a fruit, it is usually called a vegetable in cooking and regular speech because it is used in cooking.