A taisha is a grand shrine, a title borne by major shrines of great antiquity or importance, and historically by the shrine standing at the head of a network of affiliated shrines across the country.
The head of a cult
Many kami are worshipped not at one shrine but at hundreds or thousands, spread by the founding of branch shrines that carry the deity to new places, and the taisha is typically the original and senior shrine of such a network. Izumo Taisha, one of the oldest and most important shrines in Japan, and the Fushimi Inari Taisha at the head of the vast Inari cult are the classic examples, each the mother shrine from which a deity was distributed across the land. The title thus marks not only antiquity but a position of seniority over affiliated shrines.
An old word, later widened
Before the modern period the designation taisha was applied to only a small number of the very greatest shrines, Izumo pre-eminent among them. When the shrine grading of the imperial state was abolished after the Second World War the restriction lapsed, and a larger number of major shrines took or kept the title, so that taisha is now borne more widely than it once was. The word continues to signal a shrine of the first importance, whether by its antiquity, its scale, or its standing at the head of a network of the shrines that share its kami.