Call something seminal when it's so original, so groundbreaking and awesome that it will influence everything that comes after it. Picasso produced more than a few seminal works of art, for example.
Technically, seminal means something related to semen or seeds. But these days people more often use the word to describe something that plants the seed for creative growth. An innovative piece of music or literature, a fresh new idea, or an invention that changes everything could each be called seminal. Synonyms include critical, fundamental, original, and primary.