This article is from the Fencing FAQ, by Morgan Burke with numerous contributions by others.
According to Article t.7 (old 10) of the FIE rules of competition,
"the attack is the initial offensive action made by extending the
arm and continuously threatening the opponent's target."
A threatening weapon is normally interpreted to be one that will
or could hit the opponent if no defensive action is taken. In
other words, a weapon threatens if it is moving towards the
target in a smooth, unbroken trajectory. This trajectory can be
curved, especially if the attack is indirect, compound, or
involves a cutting action. Hesitations and movements of the
blade away from the target will usually be perceived as a break
in the attack or a preparation of the attack.
One common misconception is that a straight or straightening arm
is required to assert the attack. However, a straight arm is not
an attack, but a point-in-line. The attack begins
when the arm begins extending, not once it is fully extended. It
is not even necessary that the arm become fully straight,
although that is normal for attacks at medium and longer
distances. Retraction of the arm, however, will usually be
interpreted as a break in the attack.
Another common misconception is that an attack does not threaten
unless the blade is aimed at the target. This is not generally
true. The definition of an attack is the same for cuts and
thrusts, so cuts and cut-like actions (including coupe's and
"flicks") must threaten while the blade
is still out of line. Generally, an attack threatens if it is
moving towards the target as part of a smooth, unbroken movement,
regardless of where the point is located when that movement begins.
Many fencers are under the mistaken impression that a bent arm or
out-of-line point constitutes a preparation, and therefore that
they can rightfully attack into it. If the bent arm is extending
and the out-of-line point is moving towards the target, however,
this assumption is usually false under modern fencing
conventions. A successful attack on the preparation must clearly
precede the opponent's initiation of his final movement, or else
arrive a fencing time ahead of his touch.
Sabre fencers must also consider Article t.75 (old 417) of the
Rules of Competition, which states when the attack must land
relative to the footfalls of a lunge, advance-lunge, (and fleche,
historically). Attacks that arrive after the prescribed footfall
are deemed continuations, and do not have right-of-way over the
counter-attack. Sabre fencers must also remember that whip-over
touches can be interpreted as remises, and not mal-pare's.