A teneral is a dragonfly (or other insect) that has just undergone moulting. In photo below, a dragonfly has just emerged from its last larval stage to become a winged adult. It has given up the underwater existence it's known all its life to become an air-breathing, flying adult. It doesn't moult again.
The teneral pictured above is a male Western Pondhawk. You can tell it's a male by the presence of hamules, the bumpy structure that projects downward near the base of the abdomen. The hamules are used to "latch" the male and female together during mating in the wheel position. In a female, the underside of the abdomen near the base is smooth.
It's always important to know which sex you're looking at when identifying dragonflies, but with this species it's particularly helpful because both sexes start off brilliant green. Female pondhawks stay green as they mature, but the males turn from green to blue in stages starting at the tip of the abdomen and progressing toward the head. The body and eyes of the adult male are solid blue, only the face remains green – in fact, the green face is a good distinguishing feature to separate male Western Pondhawks from male Blue Dashers, which have a white face.
Tenerals have a "fresh" look to them, with distinctively vivid colors. The wings of a teneral look almost like cellophane. In fact, the wings haven't completely hardened at this stage of a dragonfly's life, and as a result the flight of tenerals is usually quite weak and slow – almost like the fluttering flight of winged termites. In fact, it's often possible to recognize a teneral from some distance because of its characteristic flight behavior. When a dragonfly is fully mature, however, its flight becomes strong and fast, as you would expect for a top predator.