Defense occurs in two ways: by Steel, and by Measure.
(That is, either you interpose your blade, or you get out of reach.)
In fencing, we call defending with steel a Parry.
Parries come in four kinds:
Each parry is named after the line number in which it ends (i.e. 1st, 2nd, 3rd, or 4th).
From the defender's engagement in 2nd.
From the defender's engagement in 3rd.
(Crouching down somewhat with the parry.)
From the defender's engagement in 2nd. With a retreat.
Riposte along the blade (i.e. by Glide)
As required by measure. Arm extends first, as always.