A seminal work [semen = seed (from the Latin seminalis)] is a work from which other works come--it is an engendering work which is so important in its ideas or technique that other people take these up and create new works too.
For example, Darwin's 1859 Origin of Species is a seminal work. Applications of evolutionary theory in art, literature, anthropology, and many other fields began to multiply after 1859 and continue to do so today. So this book was seminal.
A seminal work is related to, but should not be confused with a Magnum opus, which can also refer to the greatest work of a single individual (whether seminal or not). Origin of Species is considered Darwin's magnum opus and happens to be a seminal work too. Picasso's Demoiselles d'Avignon may be described truly as a seminal work--for it is considered the beginning point of Cubism. But Picasso's Guernica is considered by many his magnum opus -- his largest or greatest work.