BACKFILL:  The repositioning of the soil after construction of a pool.

BACKWASH:  The process of thoroughly cleaning the filter medium and/or elements by reversing the flow of water through the filter to waste.

BACTERIA:  From a health perspective, the most dangerous micro-organisms which may be living in the pool water.  Some are pathogens, which can cause infectious diseases.

BACTERICIDE:  Kills bacteria.  Chlorine is a bactericide and germicide.  Silver algaecides are actually more bactericide, and are useful on pink "algae."

BALANCED WATER:  Balanced water is the result when all of your chemical parameters are where they should be and thus balance each other.  The key components of water balance are pH, Total Alkalinity, Calcium Hardness, and Temperature; as measured using the Langelier Index of water balance.

BALL VALVE:  A device with a hollowed out ball inside which can be turned with an external handle to decrease or increase flow.

BASE:  Those chemicals of alkaline nature which will counteract the pH of an acid eventually neutralizing at 7.0. Common bases used around the pool would include Soda Ash, Sodium Bicarbonate, Sodium Carbonate, and Sodium Sesquicarbonate.

BASE DEMAND:  A titration test used to determine proper amounts of base (pH increaser) to reach correct levels.  For example, to raise pH from 7.2 - 7.6, your water may need 2 cups of soda ash.

BIGUANIDES:  The name for a certain class of sanitizers using the polymer PHMB, the only non-halogen sanitizer available for pool and spa use.  "Soft Swim" and "Baquacil" are manufacturers of this technology.
 

BICARBONATE: An intermediate form in the deprotonation of carbonic acid.

BLOWER:  Plumbed into the spa return line, air is injected to produce fun bubbles and a hydrotherapy effect in the spa.

BOOSTER PUMP: Secondary to the filter pump, a booster pump is used to power an automatic pool cleaner such as Polaris or Letro.

BREAKPOINT CHLORINATION: When you shock your pool, the goal is to reach a high enough level of free-chlorine, measured in ppm, to break apart molecular bonds; specifically the combined chlorine molecules. When breakpoint is reached with sufficient additions of chlorine, everything in the pool is oxidized.

BROMAMINES: A combined bromine - ammonia molecule. Unlike chloramines, which are strong smelling and offer no sanitizing properties, bromamine compounds continue to sanitize.

BROMINE: A member of the halogen family, commonly used as a sanitizer in spas, because of its resistance to hot water with rapid pH fluctuations.

BTU: British Thermal Unit. A unit of measurement for the use of gas by a gas appliance. Pool heaters are rated by their consumption.

BUFFER: A base such as Sodium Bicarbonate (Baking Soda), added to your pool will increase alkalinity which increases the buffering capacity of the pool; your pool's resistance to pH change.

BUFFERING CAPACITY: The ability of the pool to resist changes in pH, which prevents water balance. The buffering capacity is given by the alkalinity, a close cousin to pH. If your pH bounces, or resumes previous levels soon after adjustment your buffering capacity is too low. Check your total alkalinity.