Glossary

Heart rate variability (HRV) is the slight variation in time between heartbeats. It's a measure of how well your autonomic nervous system balances the "fight-or-flight" and "rest-and-digest" responses.

The holographic principle is a property of string theories and a supposed property of quantum gravity that states that the description of a volume of space can be thought of as encoded on a lower-dimensional boundary to the region – such as a light-like boundary like a gravitational horizon.
Read an article in Scientific American here.