Advance / Retreat
In fencing, fencers don’t normally cross their feet when moving. Instead they make sort of shuffle steps where the lead foot stays in the front, and the trailing foot stays behind. Forward steps are sometimes called advances, and backward steps are sometimes called retreats.
The fencer on the left is advancing. The fencer on the right is retreating.
A lunge is simply a large step forward made with the front foot generally made in order to reach the opponent with an attack.
Step lunge / Advance-Lunge
In fencing, a step lunge is often thought of as an action unto itself, but really it’s just a step forward and lunge executed smoothly together.
A flèche is kind of the beginning of a normal running step. The back leg comes forward and the front leg pushes backwards, generally combined with an attack. Normally, multiple running steps forward wouldn’t be considered a flèche – just the initial burst forward and attempt to touch.