... Aussiefly's Dictionary of fly fishing terms

Fly Fishing Glossary
This is a every growing list of Terms used in Fly Fishing, feel free to add to the list by contacting Uncle Barry

I
Imitator - a fly to imitate the real insect
Imitative flies - flies tied to more closely match specific insects Imitative flies are most effective in slow-moving, clear water, with finicky trout in streams with large populations of aquatic insects Impressionistic flies - flies tied to loosely suggest a variety of insects or insect families
Invertebrate - A creature that has no backbone, example, an insect or a worm
Indicator - floating object placed on the leader or end of the fly line to "indicate" the take of the fly by a fish or to indicate the path of the drift of the fly; used when nymph fishing with a slack line
Intermediate line - line with a slow controlled sink rate

K
Knotless Tapered Leader - a fly fishing leader entirely constructed from a single piece of monofilament Extrusion, or acid immersion are most commonly used to taper the leader
Knotted Leader - fly fishing leader constructed by knotting sections of different diameter leader material to each other to make a tapered leader. Most commonly used knots to construct such a leader are blood (or barrel) knot and surgeon's knot
Kype - hooked jaw of a salmon
Kevlar - a man made material, ideal for leaders/tippets for fish with sharp teeth, hard to cut
Krill - Small, shrimp like crustaceans, of the family Euphausiidae, that form a part of the marine food chain

L
Larva - the immature, aquatic, growing stage of the caddis and some other insects. Most species of caddis larva build a protective covering of fine gravel or debris to protect them in this stage. The larva is a bottom dwelling non-swimming stage of the insect
Leader - the section of monofilament line between the fly line and the fly. It is usually tapered, so that it will deliver the fly softly and away, forward of,from the fly line
Leader Material - clear nylon or other type of monofilament, fluorocarbon,kevlar, The two types most commonly used are clear nylon and monofilament
Lie - area of the river where the fish tend to sit and wait for food
Level Line - an untapered fly line, usually floating. It is difficult to cast, a poor line for delicacy or distance, and a poor choice for an all round line
Loading the Rod - phrase used to describe the bend put in the rod by the weight of the line as it travels through the air during the cast
Loch-style fishing - Stillwater flyfishing with teams of wet flies cast a short distance downwind from a drifting boat
Lure - large wet fly to imitate a small baitfish