I think even obligate carnivores animals (such as cats) consume small amounts of plant matter... if nothing else, from the semi-digested remains of the stomach matter in their prey.
And even the most herbivorous of herbivores (eg, the giant panda comes to mind) occasionally wind up consuming small amounts of animal matter... if nothing else, in the form of small insects that happen to be on the leaves/grass/bamboo that they stay busy stuffing into their mouths 10+ hours per day!
As for terminology describing various levels of carnivores, although it was years ago, I vaguely recall reading somewhere where carnivores were sub-classed according to the percentage of their diet that NEEDED to come from meat or animal products.
Hyper-carnivores were those that necessarily derived something like 70% or more of their energy and nutritional requirements from meat and other animal products (eggs/milk/etc); Meso-carnivores get something like 30% to 70% of their energy and nutrition from meat or animal products. Hypo-carnivores derive less than 30% of energy and nutritional requirements from meat or other animal products.
Omnivores, by definition, are ABLE to derive the vast majority of their energy and nutritional requirements from EITHER plant matter or animal matter...and apparently can switch back and forth quite easily.
True herbivores not only don't NEED any animal matter in their diets, they may not even be able to digest it even if they occasionally consume some.
And, obligate carnivores apparently cannot digest plant matter even if they occasionally consume some. For example, cats (obligate carnivores) will occasionally be seen eating grass but it is because the grass helps them regurgitate other matter that may be causing digestive upset.
By definition then, I think humans (and all mammals including cows and deer) start life as hyper-carnivores... fulfilling pretty much 100% of our nutritional needs via milk!
Since mammalian "herbivores" are all born with the ability to digest milk (an animal product) and obtain nutrients from it, I'm never surprised when I hear about something like a dear being seen chowing down on a dead bird or rabbits eating their young. I rather suspect that the ability to digest animal products remains intact, albeit somewhat diminished in the most herbivorous of mammals.
(BTW, They always say that a mother rabbit that eats her young is reacting to external stress but I wonder if possibly the doe is simply so nutritionally depleted by the birth process that she turns to the only source of meat available to her in order to quickly replenish the nutrients she needs!)
As for the difference between vegetarians/vegans. I'm okay with using the term vegetarian to describe a human who chooses to derive most of his/her energy/nutrition from plant matter but supplements that with animal products such as milk, eggs and honey (none of which requires that the producing animal be killed) and reserving the term "vegan" for those who refuse to knowingly consume any animal product. I'm pretty sure the vegans wind up occasionally eating some insect parts in their cereals but I'm willing to respect their right to NOT knowingly consume any animal product and recognize that this definitely differentiate them from "vegetarians" who willingly eat milk/cheese/eggs or whatever.
Pretty sure that, regardless of what humans choose to eat, we all remain omnivores... ABLE to derive MOST of our energy/nutritional needs from either plants or animals.
Vegans CAN obtain MOST of their nutritional needs from plants however, to remain healthy, they MUST supplement their diets with a few nutrients that are simply not bio-available in plant matter. I've sometimes wondered if the reason that people in third world countries eating essentially vegan diets still manage to remain reasonably healthy without supplements is that their "gathered foods" tend to be much more highly "contaminated" with insect parts and other bits of animal matter than the super-cleaned processed-foods that vegans in the US and other developed countries eat.
Just some thoughts.