Odonata is an order of insects, encompassing dragonflies (Anisoptera) and damselflies (Zygoptera). The word dragonfly is also sometimes used to refer to all Odonata. The term odonate has been coined to provide an English name for the group as a whole, but is not in common usage; most Odonata enthusiasts avoid ambiguity by using the term true dragonfly, or simply Anisoptera, when referring to just the Anisoptera.

The largest living odonates are the giant Central American damselfly Megaloprepus coerulatus, and the Giant Hawaiian Darner (Anax strenuus), a dragonfly endemic to the Hawaiian Islands. The prehistoric "giant dragonflies" belonged to the Protodonata (or Meganisoptera), closely related to true dragonflies but not part of the Odonata in the restricted sense.

Pyrrhosoma is a genus of damselfly in family Coenagrionidae. It contains the following species:

Damselflies (suborder Zygoptera) are insects in the order Odonata. Damselflies are similar to dragonflies, but the adults can be differentiated by the fact that the wings of most damselflies are held along, and parallel to, the body when at rest. Furthermore, the hindwing of the damselfly is essentially similar to the forewing, while the hindwing of the dragonfly broadens near the base, caudal to the connecting point at the body. Damselflies are also usually smaller, weaker fliers than dragonflies, and their eyes are separated.

Zygoptera comes from the Greek zygo meaning joined or paired and ptera meaning wings. They have two pairs of similar wings, unlike the dragonflies (suborder Anisoptera), whose hind wings are broader than their forewings.

Damselflies undergo incomplete metamorphosis, with an aquatic nymph stage. The female lays eggs in water, sometimes in underwater vegetation, or high in trees in bromeliads and other water-filled cavities. Nymphs are carnivorous, feeding on daphnia, mosquito larvae, and various other small aquatic organisms. The gills of damselfly nymphs are large and external, resembling three fins at the end of the abdomen. After moulting several times, the winged adult emerges and eats flies, mosquitoes, and other small insects. Some of the larger tropical species are known to feed on spiders, hovering near the web and plucking the spider from its nest.

The name Palaeoptera has been traditionally applied to those primitive groups of winged insects (most of them extinct) that lacked the ability to fold the wings back over the abdomen as characterizes the Neoptera. While this may not sound like a fundamental distinction, the complexities of the wing-folding mechanism, as well as the mechanical operation of the wings in flight (indirect flight muscles), are such that it clearly indicates the Neoptera are a monophyletic lineage. The Diaphanopterodea, which are palaeopteran insects, had independently and uniquely evolved a different wing-folding mechanism.

The problem is that the plesiomorphic absence of wing-folding does not necessarily mean the Palaeoptera form a natural group – they may simply be an assemblage containing all insects, closely related or not, that "are not Neoptera". As it stands, the relationship of the two living Paleopteran groups – Ephemeroptera (mayflies) and Odonata (dragonflies and damselflies) – to the Neoptera has not been resolved yet; there are three competing main hypotheses with many variations. In two of these – those that treat the ephemeropteran or the odonatan lineage as closer to the Neoptera than to the other "palaeopterans" –, the Paleoptera appear to be paraphyletic and thus an artificial grouping that is best abandoned.