EFFICACY:  The power to produce an effect.  Chlorine's efficacy is affected by many factors, including the sun, water balance, and the water's chlorine demand.

EFFLUENT:  The water that flows out of the pump on its way through the filter, heating, treating  equipment, and then returning to the pool.  Also known as the pressure side.

ELBOW:  A 90 or 45 degree plumbing fitting.  Used where your pipes take a turn.

electrolysis: A method of separating chemically bonded elements and compounds by passing an electric current through them.

ENZYMES:  Used in swimming pool formulations designed to break down and digest oils in a pool or spa similar to the way enzymes are used in oil spill clean-up efforts.