Columbium and niobium are synonymous names for the chemical element with atomic number 41.
In 1801 the English chemist Charles Hatchett reported a new element similar to tantalum and named it columbium (with the chemical symbol ‘Cb’). However, in 1846 the German chemist Heinrich Rose reported a new element and named it niobium. Although columbium and niobium were proven to be the same element in 1866 both names continued to be widely used for many years.
In 1950 the International Union of Pure and Applied Chemistry (IUPAC) officially designated the name to be niobium (Nb), although in the United States the element is often still called columbium.
Further information concerning the discovery of niobium is available here.