A word for the surface under an airplane began as a brand name.
Tarmac is short for tar-macadam, which Etymonline attests in 1882. Macadam is an eponym: it comes from John Loudon McAdam, the Scottish engineer who pioneered roads of compacted crushed stone in the 1820s, according to Wikipedia. Etymonline's macadam entry gives his dates as 1756 to 1836.
McAdam himself never used tar. Wikipedia notes he supplied coke to Britain's first coal tar factory but never advocated tar as a binder, preferring free-draining materials. His roads had a weakness: they rutted and threw up dust. Etymonline says the idea of mixing tar into gravel dates from the 1880s, decades after he built his roads.
The commercial version belongs to Edgar Purnell Hooley, a Welsh-born county surveyor. Grace's Guide and Wikipedia both tell the same story. In 1901, near the Denby ironworks in Derbyshire, he noticed a smooth, dust-free, unrutted stretch of road where spilled tar had been covered with slag. Wikipedia points out that Hooley heard about the spill secondhand, so the details rest on that report.
In 1902 Hooley took out a British patent for coating broken slag or macadam with tar for road making. Wikipedia describes a process that mixed tar and aggregate before laying and compacting it with a steamroller. By June 1903 he had formed a company, the Tar Macadam Syndicate, which Grace's Guide names as the origin of today's Tarmac Group.
Etymonline dates the word to 1903, when it was a trademark name. Wikipedia and Tarmac's own company history both say Hooley registered "tarmac" as a trademark, though neither gives the exact date, and Grace's Guide does not mention it.
The brand name then escaped into general use. Etymonline records that by 1919 the word was used broadly in Great Britain to mean "runway." Wikipedia adds that people now use it for paved airport areas, especially the apron near terminals, though those areas are often concrete.