These are terms I've run into while learning weight training whose meanings were not immediately obvious.
Concentric Contraction: Shortening a muscle by contracting it. A bicep curl is a concentric contraction.
Eccentric (Negative) Contraction: Lengthening a muscle under load. Lowering the barbell to your chest in a bench press is an eccentric contraction (for your chest).
Isometric Contraction: Contracting a muscle without significant movement. Your entire core works this way; even if they're not doing the main work of an exercise, they might be doing isometric contraction.
Compound Movement: Exercises that involve more than one joint and muscle group.
Isolation Movement: Exercises that involve only one joint and a limited number of muscle groups.
Main Exercise: The main exercise for a workout.
Supplemental Exercise: A secondary exercise meant to bolster or support the main exercise.
Accessory/Auxiliary Exercise: Exercises performed after the main and supplemental exercises for work capacity, antagonist muscle training (the opposite of the main muscle groups you just trained), hypertrophy, prehab, rehab, and support of the main lifts.
Plyometrics: A type of exercise that involves a rapid eccentric contraction followed by explosive concentric contraction. Used to increase power and speed. Most commonly, jumping exercises. Have to be careful with these since it's easy to injure yourself.
Midline: An imaginary line in kinesiology separating the right half of the body from the left.
Flexion/Extension: Movements that affect the angle between two parts of the body. Flexion movements decrease the angle, and extension movements increase the angle. Bending the elbow towards you in a bicep curl is a flexion movement. Straightening your elbow in a tricep pushdown is extension.
Supination/Pronation: Rotation of the foot or forearm such that the sole or palm face anteriorly (supination) or posteriorly (pronation). So, a pronated grip on a pull-up would be with your palms facing backward.
Adduction/Abduction: Motions that move a structure away from or toward the center of the body. Abduction is away from the midline, and adduction is towards it.
Internal/External Rotation (Medial/Lateral): Internal rotation is rotation of an appendage towards the midline. External is rotation away from the midline.
Open Chain: Open-chain exercises are physical movements in which the hands or feet are not in a fixed position with an immobile object. A lat pulldown is an open-chain exercise.
Closed Chain: Closed-chain exercises are physical movements in which the hands or feet are in a fixed position with an immobile object. A pull-up is a closed-chain exercise.
ATP: Adenosine triphosphate. The scientific term for the body's energy. It's an enzyme responsible for transporting energy in all cellular processes in the body. Strength training increases both the amount of ATP in the body and the body's ability to produce ATP.
Creatine Phosphate: Creatine phosphate can "reactivate" previously stored ATP. The body naturally has some of this, but creatine supplements are more efficient and really cheap.
Glycolosis: The metabolic pathway that turns glucose into ATP.
Glycogen: Glucose stored in muscles.
Hypertrophy: An increase in muscle size.
Atrophy: A decrease in muscle size.
Anabolism: The metabolic process that leads to molecular growth. In strength training, when your body is in an anabolic state, muscle mass increases.
Catabolism: The metabolic process that leads to molecular breakdown. In strength training, when your body is chronically in a catabolic state, muscle mass decreases. Cortisol (the stress hormone) can cause catabolism. Catabolism is only harmful when chronic.
Delayed Onset Muscle Soreness (DOMS): Muscular soreness due to inflammation that often occurs 1-2 days after unaccustomed exercise. Also called "myositis."