Definitions: Adapter and Fitting Types

Adapter: A fitting designed to connect different types of hose, tubing or pipe together.
Bulkhead Fitting: Fitting designed to be mounted in a wall or bulkhead and to allow tube connection on either side of barrier.
Cap: Designed to attach over the outside of a tube end to dead-end the flow. Attachment techniques include welding, push-fit, brazing, adhesives, and threads depending on application and material.
Compression Fitting: A compression fitting tightens down a sleeve or ferrule over a joint to prevent leakage.
Coupling / Connector / Hose Mender: Fitting designed to join two pieces of tubing or hose.
Coupling - Flexible: Coupling with some amount of flexibility; does not include true expansion fittings.
Cross / 4-Way: Fitting with four connection ports, typically in the shape of a cross.
Crimp Socket: Sleeve or socket to be crimped over a hose end to attach it to a fitting.
Elbow - 90°: Allows for a 90° change in direction, frequently includes a curve or radius. Elbows are also called "Ells".
Elbow - 45°: Elbow configured for a 45° change in direction.
Elbow - Other Angle: Any other angle elbow, such as 30° or 60°.
Elbow - Reducing: Joins two pieces of pipe (usually 90°) of different sizes.
Expansion Joint / Coupling: Expansion joints and couplings connect sections of tube and provide allowance for movement due to service load, shock or thermal cycles.
Ferrule: Sleeve-type fitting that slips over two ends to join them. Ferrules can be compressible and used as the sealing component of a compression fitting. They are also used as weldable connection fittings.
Gland: The male component of a tube connection, typically mating with a socket to seal or connect tubing.
Nipple: Common description of a short, straight connector fitting, typically with male threaded ends.
Offset: Fitting between runs of tube that are parallel but not aligned.
Plug: Designed to insert into the end of hose or tubing to dead-end the flow.
Reducer - Concentric: Fitting with different size ends to mate hose or tubing of different diameter; concentric reducers join tube sections on the same axis.
Reducer - Eccentric: Fitting with different size ends to mate hose or tubing of different diameter; eccentric reducers join tube sections that are parallel but not aligned.
Return / Trap: Most commonly in a 180° "U" shape. These fittings can be used to reverse tube flow direction or combined as a trap for gas or odors.
Side Outlet - Elbow: Elbow with an additional outlet at the corner perpendicular to the plane of the elbow for a total of three outlets.
Side Outlet - Tee: Tee with an additional outlet at the junction perpendicular to the plane of the tee for a total of four outlets.
Tee - Equal: Three-port fitting in the shape of a "T". Standard configuration ("Equal") indicates that the straight-through path (typically called the "run") and the perpendicular section ("branch") all have the same size ports.
Tee - Reducing: Typically, this describes a tee fitting in which the branch port is smaller than the ports of the run; it may also include size reduction from one of the run ports to the other.
Tee - Bullhead: Tee fitting in which the branch port is larger than the run ports.
Union: Three-part fitting that connects two pieces of tube and allows for disconnection without cutting.
Wye - Equal: Three-port fitting in the general shape of the letter "Y" or in which a branch departs the run at an angle other than 90°. The designation "Equal" refers to the three outlets being the same size.
Wye - Reducing: Wye for which the branch outlet is smaller than the run outlets.