Funny this. Never thought about it much, but I’d definitely say that “SSD” is a bit of a fuzzy term. The first two letters simply signify that it’s “solid-state” storage, e.g there’s no moving parts. The “drive” bit to me as a non-native speaker is a weird term.
eMMC on the other hand is a very well specified thing. It’s an “embedded” form of MMC, which is an old memory card format. The embeddedness means it’s soldered onto the PCB, but otherwise the computer interacts with eMMC in the same protocol as with MMC.
Under the hood eMMC is just NAND storage or some evolution thereof plus a “controller” that translates the MMC protocol to signals that control the underlying NAND storage. I think generally the storage and controller would live on the same chip, but don’t take my word on that.
SSD on the other hand seems to be much more an umbrella term. Depending on it’s form factor (SATA, M.2, PCIe…) it can support different protocols. The storage will generally be some form or evolution of NAND. Generally the storage chip(s) and the controller are separate, but I’m not sure whether that’s a requirement.
My first instinct was to define the “drive” part of Solid State Drive to mean something as much as “it’s removable”. But that leaves me with the dilemma: would we call microSD an SSD? MMC? CompactFlash? In practice I wouldn’t, so that’s not it. Looking at the wikipedia article about the difference between SSD and memory cards, I’m also getting nothing tangible. Rather they just claim they serve different markets and have different use-cases. Nothing of that is inherent to the organisation of the technology though.
So I’m going to conclude that SSD is a vaguely or arbitrarily scoped term. In every day speaking I’d agree that I wouldn’t use it to describe embedded storage inside smartphones or the likes, but I wouldn’t be able to give you any better answer than “yeah that’s just convention”. On the other hand, I think eMMC is the accurate term here, unless there’s a successor of it that I haven’t heard of yet that just so happens to be included with the FP4. 
Thanks, cloudy with clear patches. Variable with constant abbreviations that shift with the winds of evolving matter.
I didn’t expect such an informed view, but thank you for shining some light in this grey area.
Trip down memory lane inc 
I too believe the term SSD was just coined to point out it doesn’t have the moving parts, like a (mechanical) HDD. Back in the days it was not much more than flash memory with a SATA (or even PATA!) interface.
I remember buying a SSD in 2008 for in a ThinkPad laptop (it was terrible but much better than the replacement), and I had to phone them up regarding the order. Dude kept referring to the thing as ‘memory’. And I mean not by mistake; consistently.
I had a PATA SSD in my firewall (a P1 200 MHz or something like that), it was small, and it replaced an old even slower mechanical HDD.
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