The words we use, defined the way we use them — including where a number is an estimate rather than a measurement.

Readiness — A daily composite of resting heart rate, HRV, temperature deviation, respiratory rate and last night's sleep. Read the contributors, not the score.

HRV — Heart-rate variability — the variation between consecutive heartbeats. Generally higher means better recovered, but the useful signal is your own baseline, not a population average.

SpO₂ — Estimated blood oxygen saturation. A consumer optical sensor gives a trend, not a clinical measurement.

Strain — Accumulated cardiovascular load for the day, derived from heart rate over time.

Sleep stages — Light, deep and REM bands inferred from movement, heart rate and HRV. An estimate with useful week-to-week trend value.

Electrochromic — A lens that darkens when a small voltage is applied. You control it, and it works behind a car windscreen.

Photochromic — A lens that darkens in response to UV. It handles itself, but does not react behind UV-filtering glass.

TAC — Laminated polarizing film. Light and inexpensive, used for sunglasses.

CR-39 — The standard optical plastic for prescription lenses.

IP68 — Dust tight and protected against continuous immersion in fresh water under test conditions. Not the same as a swimming rating.