1. The aquatic stage of an ephemerid
2. Artificial imitations of all aquatic insect life
3. A small fairy-like woman at the bottom of your garden

NB much confusion can occur when dealing with the nymph and wetfly terminology. If you find yourself in this state of confusion, it helps to remember that the true definition is indeed number 3 and that by spending time at the bottom of the garden, talking to your nymphs, you will discover a certain purity of mind.
Am I helping you here?