I've been using the phrase, optimal exposure, as meaning the maximum exposure within my needs for depth of field and rendering of movement that does not blow out important highlights.
Optimal, is an adjective often understood as meaning the best or most advantageous. Proper, is an adjective often understood as meaning suitable or acceptable.
Some use proper as synonymous for correct or right. In that context, if a person believes there is only one correct or right answer, proper could be viewed as synonymous with optimal. Personally, I use the term in the former context: his conduct was not perfect but it was proper.
In many people's usage - including mine - proper is not synonymous with optimal.
Truth be told, I sometimes strive to achieve a proper exposure. In other words, I'm not always seeking the best exposure. It's not always a priority to maximize exposure within my creative goals for a photo. I often seek an acceptable exposure within the constraints of my creative objectives.
The difference may be as little as 1/3 to 2/3 stop of light, but it's measurable and, in the opinion of at least some, of enough significance to merit a distinction between an optimal and proper exposure.
I would slightly modify the above-quoted definition to read: a proper exposure is one that produces acceptable noise, depth of field and motion blur without blowing out important highlights.
In other words it's an exposure that puts enough light on the sensor that noise visibility in the photo will be acceptable, that doesn't put so much light on the sensor that important highlight details are irrecoverlable, and that meets my needs for depth of field and rendering of movement.
It's subjective - perhaps, more a characterization than a definition - but I have difficulty separating the photographer's personal aesthetic and goals from the pursuit of an acceptable or optimal exposure.