Llewellyn Hilleth Thomas (31 October 1903- 20 April 1992) was an Anglo-American physicist and applied mathematician. He is best known for his contributions to atomic physics, in particular:
Born in London, he studied at Cambridge University, receiving his BA PhD, and MA degrees in 1924, 1927 and 1928 respectively. He proposed Thomas Precession in 1926, to explain the difference between predictions made by spin-orbit coupling theory and experimental observations.
In 1929 he obtained a job as a professor of physics at the University of Ohio, where he stayed until 1943. He married Naomi Estelle Frech in 1933. From 1943 until 1945 he worked on ballistics at the Aberdeen Proving Ground in Maryland. In 1946 he became a member of the staff of the Watson Scientific Computing Laboratory at Columbia University, remaining there until 1968. In 1958 he was elected as a member of the National Academy of Sciences. He was appointed visiting professor at North Carolina State University in 1968, retiring from this position in 1976. He died in Raleigh, North Carolina.