SFP Small Form-factor Pluggable
A standardized pluggable transceiver for network gear. It lets the same switch port become fibre, copper or a direct-attach cable depending on which module you insert.
The family grows with speed: SFP (1 Gbit/s), SFP+ (10), SFP28 (25), QSFP+ (40), QSFP28 (100) and onward. The module also determines reach and wavelength — SX for multimode over hundreds of metres, LX and ER for singlemode over tens of kilometres, BiDi which manages on a single fibre. For short in-rack runs, DAC cables with fixed module ends are cheaper and draw less power than optics. The irritating part is vendor lock-in: the switch reads an EEPROM in the module and may refuse third-party optics. Some brands have a flag to skip the check, others require the module to be recoded.