This is a tricky subject.
There's no doubt in my mind that there is room for improvement, but what to improve, and how?
We have to remember that 5E is trying to be the bridge between editions, that means some jargon must remain in place, regardless of how meaningful it is.
Armour Class for example, lost its original meaning long ago. As far as I understand it, the meaning changed from 'armour class' to 'defense value' when DND stopped using hit tables and started using THAC0.
In order for 5E to be a refining of core DND concepts, terms like Armour Class and Hit Points must be kept.
On the other hand; Opportunity Actions, Opportunity Attacks, and Attacks of Opportunity, could definitely be replaced. 3E uses AoO, which is a mouthful to say the least. OA actually means Opportunity Action, and 4E makes it sound as though there's going to be a wide variety of OAs available. But in the end they were all Opportunity Attacks, so that's what people think when they see OA.
I'd like to see OA (Opportunity Action) replaced with 'Reaction' or removed altogether. It only needs to exist if there's more than one type of action that can be triggered. Meanwhile, Opportunity Attack could simply be 'free attack'. My memory (and a quick skim reading) of 2E states that no such thing existed back then. The closest was the free attack you got if a character was 'fleeing' rather than 'withdrawing'. For what it's worth; I don't think either concept (OA nor AoO) is required for basic play. However, in a tactical combat module, they're a valuable mechanic to add to the depth of play.
If we're serious about trying to clean up and simplify the language of DND, a good starting place would be to draw up a table of jargon that showed how the terms varied across the editions. (It would take a lot of work to draw up that table, so maybe just a list would be better)
Once collected, each of the terms can be compared across editions, and the most self explanatory one can be chosen for 5E.
The results should then be sanity checked, so that 'burst/blast' confusion doesn't happen. 'Burst and spread' would have been better for the simple reason that they aren't both 5 letter B words.
There is a lot to be said for natural language. There isn't much confusion to be had when the spell says '20ft cube' or '30ft radius'. We know what those things are, even back at the age of 10 when I started playing DND, there was no explanation required.