The term tipper can be used as a general term of reference for hitting with the tip of any attack, but is usually used in terms of the movesets of Marth and his clones and semi-clones, as well as the Home-Run Contest.
With Marth-based movesets
"Tipper" is typically used in reference to Marth's attacks and those of characters with movesets based on his. Marth's tippers are named so due to the end of his sword being the sweet spot and being stronger than the rest of his blade. Roy features a "reverse tipper", as his sweet spot is at the base of his blade while the rest of his sword is weaker. Lucina and Chrom, as a result of not having a particular sweet spot on their swords, have no hitbox distinction between tippers/reverse tippers and other hits. Several of Corrin's key moves have a tipper sweetspot, such as all smash attacks and Dragon Lunge, though this is not to the same extent as the previously mentioned two (due to all of Marth/Roy's sword attacks, barring Counter, carrying this mechanic). Byleth's Adreadbhar also has a tipper mechanic.
Less frequently, the term is also used for other sword-wielding characters' moves which may have slight sweet spot and sour spot hitboxes; for example, both Link's dash attack and Ganondorf's Warlock Blade attack have a sweet spot on the tip of their swords.
Origin
Marth's tipper animation is a close match for the motion blur on the tip of his sword in some attacking animations in Fire Emblem: Mystery of the Emblem.
In Home-Run Contest
Beginning in Melee, the Home-Run Bat's forward smash has several hitboxes that each do a varying amount of knockback, with the ones on the handle dealing the least and the ones at the tip dealing the most. As a result, hitting a tipper in the Home-Run Contest knocks Sandbag the farthest, and is the usual method of finishing Sandbag, unless the character has an attack that is stronger such as the Warlock Punch. The tipper still exists in normal play, though the distinction is typically irrelevant, as the non-tipper hitboxes are enough for a one-hit KO regardless.
An aerial tipper is a tipper in which one hits Sandbag while it is still in the air. In Melee, it sends Sandbag farther than a normal tipper due to the mechanics of the Sakurai angle; in Brawl and SSB4, this has been changed to a fixed angle, removing the distinction.