These things have various definitions when it comes to dueling and MMA etcetera but here’s my take.

In D&D 5e you can use the “Reaction” rule to power these moves, in other systems you may allow a certain number or in the most austere cases force the character to forfeit their following action or prepare ahead of time with a readied action. Being a special maneuver, limiting the use to once per round is fine but it may make sense to allow it more depending on the system.

Parry/Deflect
A parry is using your weapon(s) in an active motion to strike and deflect an attack.

As a reaction to being attacked in melee if you’re wielding a weapon that could reasonably deflect the attack you may make an ‘opposed’ attack roll, if its higher than the enemies attack roll you block that attack. Riposte: if it is 3 or more than the opposed attack and beats their AC you strike them back with that attack roll. (Optional: require a finesse weapon in 5e.)

Block
Positioning your weapon or shield between yourself and the attackers strike to stop it.

As a reaction to being attacked in melee, if you’re wielding something that could reasonably block the attack make a strength roll (+2 with shield), if the roll is higher than the opposed attack you block it. If the roll is 5 or more than the opposed attack you may push the target back 5′.

Dodge
Ducking, moving aside, and any other avoidance maneuver to escape attacks.

As a reaction to being attacked, make a Dex check, disadvantage if the attack is ranged, if it is higher than the opposed attack that attack misses, if the roll is 3 or more you may shift (move) up to 5′.

Full Defense (optional)
Full defense is using everything in your power to move away from attacks and minimize their impact. Taking no aggressive actions whatsoever.

On you turn you mac declare Full Defense, until your following turn all attacks against you (that are not from behind if playing 5e with flanking or in systems where that is taken into account) are reduced by d6 and increase reflex saves by d6. (You must be able to move, not bound, and you must be armored or holding something you could reasonably block with.) Alternatively you can replace the d6 with the characters hit die.

My current Parry rule
In DCC I use a single parry/block/deflect reaction along with a defense as described above. “Parry: in reaction to an attack you may sacrifice one of your action dice for the next turn to add your attack bonus (with a weapon you’re wielding) to your AC vs a total number of attacks equal to your attack bonus until your next turn. (Can be used in reaction.)” Its elegant but no riposte.