In the late 19th century the luminiferous aether ("light-bearing aether") was invoked as the medium for the propagation of light, when it was discovered, from Maxwell's equations, that light is an electromagnetic wave. By analogy to mechanical waves, physicists assumed that electromagnetic waves required a medium for propagation, and hypothesized the aether.
Other than the question of propagation, the aether was intended to solve the problem that Maxwell's equations require that electromagnetic waves propagate at a fixed speed, c. As this can only occur in one reference frame according to Newtonian physics (see Galilean-Newtonian relativity), the aether was hypothesized as the absolute and unique frame of reference in which Maxwell's equations hold.
The famous Michelson-Morley experiment in 1887 contradicted the aether hypothesis, and was one of the experiments that led to the formulation of the theory of relativity in 1905. However, physicists did not fully abandon aether until the theory of relativity was fully accepted.