I have been doing some interval training and it made me think about how fast my heart rate can drop off from 160ish to "resting." It seems like fitter people would recover to resting faster, but I couldn't find a name for this.
Heart Rate Recovery (HRR).
Not a super interesting term, but perfectly descriptive. See this paper for a systematic review and meta analysis on the relationship between HRR and cardiovascular disease risk and all cause mortality. The definition for HRR provided in the introduction:
Heart rate recovery (HRR) is defined as the rate at which heart rate decreases within the following minutes after the cessation of physical exercise, and reflects the dynamic balance and coordinated interplay between parasympathetic reactivation and sympathetic withdrawal.