Open rate
The share of delivered emails recorded as opened — a weaker number than it used to be.
Opens are counted with a tiny tracking image. When it loads, the open is recorded — which means an open is really “the images loaded”, not “a person read this”.
Privacy protections have made it weaker still: some providers fetch images on the recipient’s behalf, so opens are registered for messages nobody looked at. Reported open rates rose without anything changing.
It remains useful for comparing two subject lines to the same list on the same day. It is not useful as an absolute measure, and clicks or replies are firmer ground.