１ニョッキがintegral curveでニョッキの集まり、つまりニョッキニョキがcongruenceなのか！？wikipediaによると以下の通りに書かれているんで、そんなイメージかなと思ったんですけど。
In general relativity , a congruence (more properly, a congruence of curves) is the set of integral curves of a (nowhere vanishing) vector field in a four-dimensional Lorentzian manifold which is interpreted physically as a model of spacetime . Often this manifold will be taken to be an exact or approximate solution to the Einstein field equation .