Covers any component of a character, both the meaning side, the 形旁, and the sound side, the 声旁. The one component a dictionary files the character under is the 部首, the radical: every radical is a component, but not every component is a radical.
history
Originally a pair of opposites, 偏 named the left-hand part of a character and 旁 the right-hand part. The split stopped being observed long ago, and the compound now covers a component in any position: top, bottom, corner or enclosing.
