Buff

From The StarCraft 2 Wiki

A buff is generally a non-permanent attribute added to a unit, affecting it directly or indirectly for a period of time in a positive manner. Such an effect is the StarCraft Science Vessel ability to shield units, reducing the damage they receive for a period of time, or damage dealt. It applies to a single target but it retain this "buff" even when they move around.

The direct opposite of a buff is a debuff, that works similarly, but has direct/indirect negative effects on the recipient.