Fly Fishing is a great activity but if you are new to the sport the language used can be a bit intimidating. In the list below we tried to explain some of the language commonly used.
Adult
The stage of an aquatic insect, winged, and ready to reproduce
A.F.T.M.A.
American Fishing Tackle manufaturing Association, organisation responsible for the weight class model used for flyrods, lines etc. So for example a #5 weight line or to be complete an AFTMA #5 weight line.
Each rod is built to handle a certain line weight.
Typically the first 30 feet of a line is measured to find out the number of grains in that strectch. The higher the grain, the heavier the line, the higher the weight class.
A.P.G.A.I.
Association of Professional Game Angling Instructors
Baetis
Type of Mayfly
B.W.O.
Blue Winged Olive ( fly pattern )

C.D.C.
Cul de Canard ( French for Duck bottom ), very small and light feathers typically used for dry fly patterns )
C.D.C.C.
Cul de Canard ( see above ) Caddis. A Damselfly pattern ( subgroup to the Zygoptera and member of the Odonata )
Dead drift
The situation where your line and fly ( or nympf ) travels at exactly the same speed on the current. This allows the fly to make a "drag free" drift, making your fly look more natural to the fish. Also called a perfect drift.
D.T.
Double Taper ( line )
E.F.F.A.
European Fly Fishing Association
E.H.C.
Elk Hair Caddis, Caddis tied with Elk hair
F
As in Float, when indicating a Floating line
False cast
This is the standard cast used to lengthen or shorten your line, to change direction or to dry off your fly.
It is called false as the line is not allowed to touch the surface while false casting.
F.F.F.
Federation of Fly Fishers
The Federation of Fly Fishers is a 43 year old international non-profit organization dedicated to the betterment of the sport of fly fishing through Conservation, Restoration and Education. The Federation of Fly Fishers and its Councils are the only organized advocate for fly fishers on an national and regional level.
G.R.H.E.
Gold ribbed hare's ear. A fly pattern where the body is made of hare's ear and then ribbed with "gold"
I
As in Intermediate when indicating an Intermediate line
Pause
FFF terminology used to define the pause period to allow the line to stretch after power application of the rod.
The pause starts as soon as the line passes the rod tip.
P.M.D.
Pale Morning Dunn ( Fly pattern )
Polyleader
A Polyleader is similar to a monoleader but different in the sense that the leader is tapered while the core of the leader is level to perform better as an extension of the flyline.
P.T.N.
Pheasant Tail Nympf ( Fly Pattern )
Pupa
The insect stage between a larva and an adult
Rule of "eleven" ( also see "X" )
To determine the diameter of the leader use .011 inch = 0X
If your leader is .007 inch, deduct the 7 from eleven and you will find that .007 is a 4X leader
S
As in Sink, when indicating a sinking line
Slack line
When there is no tention between fly and rod tip
S.L.T.L.
Stands for Straight Lines and Tight Loops ( standard Fly fishers greeting )
Streamer
A fly tied to simulate some sort of baitfish. Typically tied in all sizes and various colors.
Tailing loop
A casting problem that occurs when the front of the line and the leader "crashes" into the main line
near the end of the cast.
W.F.
Weight Forward ( line )
X
Used as a measurement to determine the thickness of a leader or tippet material. Typically used like 4X or 5X.
If you want to know the exact diameter you should subtract the "X" number, so for example 4X from the number eleven.
The remaining number ( in this case 7 ) is the diameter in thousands of an inch or .007"
In the metric system .007" would be 0.1777mm so roughly 0.18mm.
Also see "rule of eleven"