15mm compression x 3/4 inch female BSP coupler / adaptor.
Brass fitting, made to the European EN1254 standard.
Compression plumbing fittings do not require heat or solder. The joint is formed by tightening a nut and compressing the olive (included with fitting) onto a pipe, which forms the seal. They can be used on wet pipework, and are suitable for hot and cold systems.
BSP (British Standard Pipe Thread) sizes don’t match up with what you would measure using a ruler. Have a look at the BSPT link at the bottom of this page for more information about how plumbing threads are measured.