Glossary of Terms used in resistance training.

Rep	A repetition of the exercise you are doing, eg. 5 reps means doing the exercise 5 times in a row without a rest in between.
Set	A group of reps followed by a rest. eg 2 groups of 5 reps = 2 sets of 5. An exercise program will often list exercises to be done a X sets of Y reps
Tempo	The speed of the exercise, an exercise done slowly is done at a slow tempo. complex exercise program my split an exercise into a different tempo for different phases of the exercise (eccentric, pause, concentric pause)
Concentric	Concentric muscle contractions involve movements that shorten your muscles.
Eccentric	Eccentric exercises are exercises that involve lengthening a muscle while still contracting it.

Isometric	Exercises are exercises that involve the contraction of muscles without any movement in the surrounding joints.
Compound	An exercise that involves two or more simultaneously joint movements.
Isolated	An exercise that involves just one discernible joint movement.
Closed Chain	An exercise or movement in which the end segment of the exercised limb is fixed, or the end is supporting the weight. Most compound exercises are closed-chain movements.
Open Chain	An exercise or movement in which the end segment of the exercised limb is not fixed, or the end is not supporting the weight. Many isolated exercises are open-chain movements.
Functional	An exercise which allows one to gain motor development or strength in a manner in which it is used in the execution of a particular task (eg: specific sport skill, occupational task, or daily activity).
Hypertrophy	An increase in muscle size
DOMS	Delayed Onset Muscle Soreness
1RM	The weight where you can only lift it once for a specific exercise. 3RM and 5RM are also common measures
PR / PB	Personal Record / Personal Best

I will add to this as time permits