瑜伽師地論略纂 2.25

2.25

「一心剎那」者，意說於一境中專一性心，雖多念仍名一心，又以前後性類相似名一心。

As for "one thought-moment": this means that regarding a single object there is a mind of focused concentration, which, though lasting many moments, is still termed "one mind"; furthermore, because the nature and category of earlier and later moments are similar, it is termed "one mind".