Dressage in Definitions

Dressage	The training of a horse.
Inside	Generally used when riding, this term means to the inside of the arena or area in which you are riding. It can also mean to the side of the bend. If the horse is bent left, than the left is his inside at that moment.
Outside	Generally used when riding, this term means to the outside of the arena or area in which you are riding. It can also mean to the opposite side of the bend. If the horse is bent left, than the right is his outside at that moment.
Rhythm	the timing of the footfalls within the stride.
Self Carriage	A sense of independent balance. A horse has Self Carriage at all times - or else he would fall down! Some people refer to this as the beginings of Collection, while others may mean that the horse does not lean on the forehand or the reins.
Tempo	is the rate of repetition of the strides. It is usually measured in strides per minute. Tempo is supposed to stay the same, whether the horse is in collected, working, medium and extended gait types. The tempo is consistant, while the stride length varies.

In relation to movement . . .

Acceptance	Lack of evasion, resistance, or protest. Used in reference to the horse's willingness to become obedient to contact, the application of the aids, and the direction of the rider's weight.
Activity	Energy, vigor, liveliness – referring especially to that of the hind legs.
Balance	Relative distribution of the weight of horse and rider upon the fore and hind legs (longitudinal balance) and the left and right legs (lateral balance).
Loss of Balance	A sudden increase of weight onto the forehand and/or to one side.
Beat	This is not what a dressage whip is for; it is a footfall within a gait, such as a walk, trot or canter.
Outside	Generally used when riding, this term means to the outside of the arena or area in which you are riding. It can also mean to the opposite side of the bend. If the horse is bent left, than the right is his outside at that moment.

In relation to riding . . .

Behind the Bit	An evasion in which the horse retracts or shrinks back from the bit/contact. This can also be in response to the rider's unfeeling or uneducated hands/use of the bit.
Behind the Leg	Usually refers to the horses' lack of effort to be obedient to the leg or seat aides. This can also mean the rider is aiding the horse at the improper time (when the horse is not prepared in footfall or balance).
Transitions	Usually used in reference to the changes between gaits. Can also refer to any change of direction.