a/ (Mysterious circle) Pictured as wallpaper on this website. The seventeenth century mathematical basis  from which all footwork and distance was calculated and executed in Spanish rapier play. b/ Used in some stage combat schools footwork, for two parallel lines that join one's lead and lag foot to one's opponents lead and lag foot, respectively on a circle. See, Ten Directions of footwork.
Imbroccata
 a/ a downward thrust, generally delivered from the right with the hand pronated, over the opponents sword arm or blade. b/ Pronated thrust delivered from prime to either of the opponents shoulders or chest.
Immediate
 Without pause.
Imrov
 1/ A method of creating drama which is actor driven. Improvisation usually through character. 2/ A stage combat style practiced by those with a death wish. 
In distance
 Within striking distance. Within fencing measure.
In line
 Having the sword arm straightened with the point threatening one's opponent.
Indirect attack
 An attack or riposte to another line proceeded by a preparation in the original one. e.g. Beat, changement, thrust.
Inquarte
 See Inquartata.
Inquartata
 An offensive movement made while removing the body out of line by a side step.
Inside line
 The area of attack or defence, separated by an imaginary vertical line to the inside (Non weapon bearing side) of the sword hand. The parts of the target furthest from the attackers sword arm.
Inside measure
 Within fencing measure.
Inside Mollinello
 Mollinello executed on the inside line of the body.
Insistence
 The continuation of attack in the same line after an initial attack has been parried. (Renewed attack)
Instinctive parry
 Simple or direct parry.
Insufficient parry
 A parry which does not successfully close a line of attack, and through which the opponent can score a hit.
Intercepting the bind
 Form of pris d'fer used in double weapon fencing, such as rapier and dagger, where the second weapon catches the opponents blade during the bind and returns it to it's original position.
Int.
 (Film) Interior.
Interna linea
 Inside line.
In the round
 a/ Style of theatrical presentation with the audience on all sides b/ Combat that is not linear, such as in small sword or modern fencing.
Invalid parry
 A parry that does not sufficiently divert an incoming blade from the target area.
Invitation
 A movement intended to draw a specific attack from ones opponent. Opening a line of attack to draw a response.
Involuntary sounds
 Reactive unconscious sounds made during a fight. Can often add weight or reality to a fight.