Literally: one – child – mutual – transmit
Alternately: A traditional system where the inner, secret, or ultimate teachings of an art, craft, school of thought, etc. are passed on to exactly one heir (usually from a father to an oldest or chosen son).
Notes: This is a compound of compounds. Isshi means “one child,” as you’d expect, and souden refers to inheritance across generations.
The idea expressed in this phrase is practically a staple in martial arts stories, where interpersonal and familial drama are spiced up with (often supernatural) violence. It doesn’t strike me as a particularly bright idea if you want your “house style” to survive long-term, but what can you do.
A variant replaces 一 with 父 (“father,” pronounced here as fu), making the patrilineal transmission explicit. Reading 相 as shou in this case is considered an error.