Slayer

Has nothing to do with the anime Slayers.

Slayer is a title that gets bandied about a lot on /tg/, but the two most prominent /tg/ references are as follows.

Warhammer

In Warhammer Fantasy, dwarves who feel too ashamed to live with themselves take up a ceremonial pledge to the god Grimnir. They tattoo themselves, dye their hair red and spike it into a huge mohawk, grab the biggest weapons they can, and set off to redeem themselves through dying in battle. Of course, they can't just kill themselves; they have to fight as hard as they can and to the very best of their abilities, until they fall against a nasty legitimately too mighty for them to overcome. Sort of like a super-metal paladin.

The most famous Slayer is the titular star of Gotrek & Felix.

In Warhammer Fantasy Roleplay, Slayer is a career path that dwarves can enter, but it is a one-way trip; other than the more powerful levels of Slayer, they cannot ever change out of that career. It's a brutal combat machine, but utterly useless at anything else.

Pathfinder

In Pathfinder, the Advanced Class Guide splatbook introduced a bunch of new classes, which basically ripped off the idea behind 4th Edition Dungeons and Dragons's Hybrid Classes: each class was basically a fusion of two existing classes. The Slayer is a Rogue/Ranger hybrid, essentially a beefier rogue with the ability to select a mixture of rogue and ranger talents and the ranger's favored enemy trait matched to a weaker version of the rogue's assassination traits (Sneak Attack, Studied Target). Great if you really, really want to play a Bounty Hunter type character.