Insertion loss
Insertion Loss is defined as being the difference in power level just before (P1) and immediately after (P2) a joint or inserted optical device.

Where Insertion Loss = P1-P2 (dB or mW)

The best insertion loss figures tend toward 0 dB, meaning that the optimum amount of optical energy has been transferred across a joint or inserted optical device.

Return Loss
Return Loss refers to the loss in the power level of reflected, or returned light, compared to the incident power level, just before that reflection. The better the return loss figure across a mated connector or joint, the better chance the signal has of reaching the end of a channel with enough energy to be decoded. In addition, the better return loss figure means that subsequent signals being sent from the source are less likely to be corrupted, or overloaded, with reflected energy, which is particularily important in high speed data networks.

Ferrule
Part of the fibre optic connector in which the optical fibre is centrally positioned. The ferrules of the two connectors are inserted into the sleeve of a through-adaptor until their frontal faces touch each other. In the case of SM connectors, the ferrule front faces are available with different polishings, and are usually made of zirconium ceramic, whilst polymer ferrules are used in MM applications. The majority of connectors have a ferrule with a diameter of 2.5mm. Small-form-factor connectors, e.g. LC or MU, mini-ferrules have a diameter of 1.25mm.

Sleeve
Part of the through-adaptor in which the ferrules of the two connectors to be connected are inserted. For SM applications, they are typically made of zirconium ceramic or, in the case of MM applications, they are also often made of phosphor bronze.

Pigtail
An optical fibre, usually with a secondary coating (typically, 0.9mm in diameter), fitted at one end with a connector; it is primarily used for splicing of loose-tube fibres in racks and cabinets.

Patch Cord
An optical fibre cable fitted at both ends with connectors, usually with a diameter of 2.4 or 3.0mm, or 1.8mm in the case of SFF.