Priority
Priority is a property of every attack in the Super Smash Bros. games. In general, priority refers to an attack's ability to trump another attack if both are used at the same time. Priority is directly related to the damage the attack deals.
For example, if two Marios punch each other at the same time, the attacks will "cling" (that is, both attacks will be stopped) and neither will take damage. This is because the attacks have the same priority; this can also happen if the priority of the two attacks are similar but not completely equal. But if Mario's weak punch were to collide with Yoshi's forward smash, then Yoshi's attack (with its higher priority) will go right through Mario's attack, damaging him without Yoshi taking damage.
If two characters attack each other in the air at the same time, then they can hit each other regardless of priority (unless one of the two attacks comes out faster), though if one or both of the attack are a Special, priority still decide who gets hit. If one of the characters is on the ground and the other is in the air, priority decides who gets hit, but if it's the same priority, both trade hits. Priority only affects grounded characters.
A attacks's priority is directly related to its damage output. If one of the colliding attacks does more than 8% than the other attack, the stronger attack will out-prioritize the weaker attack. If the two colliding attacks' damage output are within 8% of each other, they will "cling". For example, if Marth's non-tipped f-smash collides with Ike's f-smash (14% vs. 22%), they will "cling". However, if Marth's f-smash collides with Ganondorf's f-smash (14% vs. 24%), Ganondorf's f-smash will out-prioritize Marth's.The main exception to this is grabbing: a grab always has more priority than an attack. When the attacks "collide", the grab overrides the attack and stops the grabbing character from flying back but still takes the damage (this does not work with Olimar since his Pikmin grab the characters and not Olimar himself). Also, sword-wielding characters generally have high priority in their sword attacks, and if the opponent out-prioritize the sword, the character still doesn't get hit (unless they hit the character instead of the sword) as the sword hitbox is disjointed.
There are some attacks that do not have priority and instead ignore the laws of priority. These attacks have no priority as they cannot "cling" with other attacks. As such, these attacks cannot be cancelled by opposing attacks but they are incable of cancelelling opposing attacks as well. One such character that this heavily affects is Meta Knight as all of his sword attacks have no priority. This, combined with how fast his attacks are, causes many people to believe that he has very high priority. However, if his sword attacks did had priority, he would have very low priority. Other such attacks with no priority include King Dedede's f-smash, Ike's u-smash and d-smash, and the Space animals' lasers. It is important to know that only projectiles and attacks with disjointed hitboxes can have no priority.
