its called a generic trademark. its when a brand name become synonymous with the product
Ah. That. I wasn't aware that Mascara/Jaccuzi were examples... I'm not even sure if they're spelt correctly as I'm just copying what the previous commenter wrote. This is, I believe, the first time I've ever typed those words in my 40 years of life.
In any case, the whole "jello" thing seems to be a regionally isolated one as far as I'm aware. Jelly is just jelly here in the UK, but I believe Americans use the generic trademark thing. I believe it is the other way around with Hoovers (i.e. What most upright vacuum cleaners get called here, but they don't share that trademark name over the pond).