Prevent is the key word. A successful parry deflects the threatening blade away from the target. Most referees are listening for the contact, giving the benefit of the doubt to the fencer making the parry, and few these days will declare a mal-paré.
If the attacker must replace the point into a threatening line before continuing, it is a remise (renewal of the attack) and does not have right-of-way over the riposte. However, if the parry does not deflect the blade, or deflects it onto another part of the target, then the attack retains the right-of-way (mal-paré by the defender). In practice, very little deflection is needed with a well-timed parry.
A well-executed parry should take the foible of the attacker’s blade with the forte and/or guard of the defender’s weapon. This provides the greatest control over the opponent’s blade, although the parry will be judged sufficient if the attacking blade is sufficiently deflected. In ambiguous cases, however, the benefit of the doubt is usually given to the fencer who used his forte/guard. Foible to forte doesn’t generally create much deflection, but in competition rules, could be interpreted as a counter-time parry or beat attack by the opposing fencer (more on that another time).
At foil, the opponent’s blade should not only be deflected away from the target, but away from off-target areas as well. An attack that is deflected off the valid target but onto invalid target can still retain right-of-way, preventing the riposte from scoring in time.
At sabre, the opponent’s blade need only be deflected away from valid target, since off-target touches do not stop the phrase. Cuts are considered parried if their forward movement is checked by a block with the blade or guard. Contact with the blade or guard may be interpreted as a parry, even if a whip-over touch results. Avoiding whip-over touches altogether requires exceptionally clean and clear parries.
At epée, a good parry is simply any one that gains enough time for the riposte. Opposition parries and binds are commonly used, since they do not release the opponent’s blade to allow a remise.