I very much concur with @Jpe61 for the origin and definition.
I would add that a synoptic display is more precisely what an aviator or other complex machine operator wants. It is a visual graphic that conveys the status/summary at a glance without having to actually focus on a read out.
One example would be the picture shown in @JohnK 's answer. A simple summary page could be done in text, but one would have to read through the text to determine the current status of any given hydraulic pump. Similar displays are used for fuel tanks.
With the number of instruments and displays in a cockpit, and all the various tasks you have to do, you need all the instruments and displays configured in a way that convey's the most relevant data in the shortest time possible in the most intuitive of ways so the pilot doesn't have to dedicate too much attention to any given display...especially if it's required elsewhere.
As another illustration, there has been some dismay among pilots shifting from analog altimeters to Digital HUD. The issue is that an analog (needle) display, once you're familiar with it, you can see where the needle is in the scale out of the corner of your eye and you don't have to actually look at it to snap out of a dive at precisely the correct altitude, but with a digital display where the numbers are constantly changing on a steep dive or climb, you actually have to focus on the display to discern the altitude. That's far less intuitive and can cause a loss of either situational awareness of other events or delay response to other activities requiring concentrated attention. (Such as might be the case while an attack/bomber is attempting to hit a target in hostile airspace --though this example is not so much a problem for commercial or private pilots.)
One final illustration. If a simple summary or status page in text was sufficient, why wouldn't you simply have a read out in text format of airspeed, altitude, compass heading, etc? You'd almost be flying a literal desk with Word, PDF, DOS screen, or paper print outs feeding you all your information. There would be no way for a flight crew to process the information in that medium in a sufficient fashion to be able to effectively fly the plane.
Though it might be easier for a random individual off the street without any training to fill in for any given pilot and be comparably effective.
Synoptic is thus the best and most precise word for the kind of instruments and medium for displaying your situation and status summaries. Synopsis being the summary data (data or situation at a glance) and optic being the visual medium for that data (presented in ideal visual format).
As for the actual definition: Webster's Fourth ed College Dictionary has
synoptic:1 of or constituting a synopsis; presenting a general view or summary
synopsis:a statement giving a brief, general review or condensation; summary
So synopsis is the summary, synoptic is the presentation of that summary.