Imbuement is a term for when an Item requires the player to choose a specific ability of the hero they are playing. Typically the item will enhance only that ability, or provide a greater boost of Stats for that ability compared to the others.
Ability imbuements are item-specific - imbuing two separate abilities using, for example, Superior Cooldown and Quicksilver Reload, will not grant both abilities the effect of both items. However, one ability can be imbued by multiple sources.