Vagal Tone

Variability in heart rate at rest that is associated with respiratory patterns. Vagal tone reflect the functioning of the vagus nerve, which is the 10th cranial nerve and a core component of the parasympathetic nervous system, which regulates heart rate in response to signals of safety and interest.

Reference:
Kok, B. E., Coffey, K. A., Cohn, M. A., Catalino, L. I., Vacharkulksemsuk, T., Algoe, S. B., ... & Fredrickson, B. L. (2013). How positive emotions build physical health perceived positive social connections account for the upward spiral between positive emotions and vagal tone. Psychological Science, 24(7), 1123-1132. doi: 10.1177/0956797612470827