A wasp is any insect of the order Hymenoptera and suborder Apocrita that is neither a bee nor an ant. The Apocrita have a common evolutionary ancestor and form a clade; wasps as a group do not form a clade, but are paraphyletic with respect to bees and ants.

The most commonly known wasps, such as yellow jackets and hornets, are in the family Vespidae and are eusocial, living together in a nest with an egg-laying queen and non-reproducing workers. Eusociality is favoured by the unusual haplodiploid system of sex determination in Hymenoptera, as it makes sisters exceptionally closely related to each other. However, the majority of wasp species are solitary, with each adult female living and breeding independently. Many of the solitary wasps are parasitoidal, meaning that they raise their young by laying eggs on or in other insects (any life stage from egg to adult). Unlike true parasites, the wasp larvae eventually kill their hosts. Solitary wasps parasitize almost every pest insect, making wasps valuable in horticulture for biological pest control of species such as whitefly in tomatoes and other crops.

Wasps first appeared in the fossil record in the Jurassic, and diversified into many surviving superfamilies by the Cretaceous. They are a successful and diverse group of insects with tens of thousands of described species; wasps have spread to all parts of the world except for the polar regions. The largest social wasp is the Asian giant hornet, at up to 5 centimetres (2.0 in) in length; among the largest solitary wasps is a group of species known as tarantula hawks, along with the giant scoliid of Indonesia (Megascolia procer). The smallest wasps are solitary chalcid wasps in the family Mymaridae, including the world’s smallest known insect, with a body length of only 0.139 mm (0.0055 in), and the smallest known flying insect, only 0.15 mm (0.0059 in) long.

Social wasps are considered pests when they become excessively common, or nest close to buildings. People are most often stung in late summer, when wasp colonies stop breeding new workers; the existing workers search for sugary foods and are more likely to come into contact with humans; if people then respond aggressively, the wasps sting. Wasp nests made in or near houses, such as in roof spaces, can present a danger as the wasps may sting if people come close to them. Stings are usually painful rather than dangerous, but in rare cases people may suffer life-threatening anaphylactic shock.