Reach

While you can generally attack only adjacent squares with a melee weapon, a weapon with reach allows you to attack enemies who are further away. In general, this also means that you can no longer attack adjacent opponents, though of course you can attack them with a different weapon, and some reach weapons still allow you to attack adjacent opponents, such as the Whip. You threaten only those spaces that you can attack with a reach weapon (for purposes such as attacks of opportunity), though again there are exceptions.

If not otherwise noted, you can make trip attempts with reach weapons even against adjacent spaces.

The quintessential reach weapon is the Spear.