Vegetables are edible plants. Technically they exclude actual fruit, although many so-called vegetables (e.g. chili peppers, tomatoes, and pumpkins) are the fruit of the plant in question. Questions about anything typically grown in a vegetable garden are on-topic for this tag.
If you're having a problem with a vegetable (leaves turning yellow, plants die, etc), use diagnosis in addition.
This tag can also be used for general questions about vegetable gardening (getting started, mulching, fertilizing, watering, etc), so please add the appropriate tags for these questions too.