A fiber optic adapter is a passive component that brings two fiber connectors face to face in precise alignment. It is also called a coupler. On a panel it is the point where a patch cord plugs in at the front and a pigtail seats at the rear, forming the transition between permanent infrastructure and flexible connection.
The ceramic or plastic split sleeve inside the adapter aligns the two connector ferrules to micron accuracy. That alignment directly determines insertion loss, and dust or fingerprints entering the adapter are the most common cause of degraded links. Unused adapters are therefore kept closed with dust caps and connector end faces are cleaned before insertion.
Selection covers connector type (LC, SC, ST), channel count (simplex or duplex) and body material. LC duplex adapters are compact and provide roughly twice the ports of SC in the same panel space. Metal-bodied adapters suit field and heavy-use points needing mechanical strength, while plastic bodies serve standard in-panel applications.
Colour coding applies to adapters too and prevents mistakes: aqua marks OM3/OM4 multimode, beige OM1/OM2 multimode, blue singlemode UPC and green singlemode APC. APC (angled polish) adapters are used only with APC connectors; inserting a UPC connector into an APC adapter causes both high loss and physical damage.