Proficiency is a Stat in Nioh.
Proficiency is the measure of your effectiveness with a specific category of Weapons. They can be viewed under the Status > Additional Statistics menu.
Using them effectively in battle improves your Proficiency which in turn earns you Samurai Skill Points to spend on Skills, to be specific, Proficiency gained is related to on-hit damage dealt, with no need to actually kill an enemy to be awarded the points. This makes it so a higher attack power as well as more resilient enemies become more efficient Proficiency farms, thus making increasing your Proficiency faster and more comfortable late game. Proficiency gained seems to be roughly ~90% of the player's dealt damage with the appropriate weapon type.
Proficiency is maxed out at 999999. As you increase Proficiency you will also unlock certain Master Missions.
500.000 Proficiency in a weapon class is also required to unlock the related Mystic Art Dojo Mission.
The following lists the amount of proficiency points for a single weapon at which you will acquire a Samurai Skill Point:
You do not get any additional Samurai Skill Points for proficiency beyond 300,000. This makes it so a maximum of 70 Samurai Skill Points can be earned through increasing every weapon type's Proficiency stat past this threshold.
Agility ♦ Attack ♦ Body ♦ Break ♦ Defense ♦ Dexterity ♦ Equipment Weight ♦ Familiarity ♦ Gold ♦ Guardian Spirit Bond ♦ Heart ♦ Ki ♦ Ki Pulse ♦ Level ♦ Life ♦ Magic ♦ Ninjutsu Capacity ♦ Ninjutsu Power ♦ Onmyo Magic Capacity ♦ Onmyo Magic Power ♦ Onmyo Magic Skill Points ♦ Parry ♦ Samurai Skill Points ♦ Skill (Stat) ♦ Spirit ♦ Stamina ♦ Strength