William Thomson, 1st Baron Kelvin (or Lord Kelvin), OM, GCVO, PC, PRS, FRSE, (26 June 1824 – 17 December 1907) was an Irish-born British mathematical physicist and engineer. At Glasgow University he did important work in the mathematical analysis of electricity and thermodynamics, and did much to unify the emerging discipline of physics in its modern form. He is widely known for developing the Kelvin scale of absolute temperature measurement. He was given the title Baron Kelvin in honour of his achievements and is therefore often described as Lord Kelvin. The title refers to the River Kelvin, which flows past his university in Glasgow, Scotland.

He also had a later career as an electric telegraph engineer and inventor, a career that propelled him into the public eye and ensured his wealth, fame and honour.