I’ve got a fun solution. You can write the definition as an image and then have that left aligned so that the text wraps around it, like in the example below. The code is just:
You walk towards the strange creature. It has six legs and a pair of tiny gossamer wings.
*image bee.png left Definition: Honey Bee Apis Mellifera, A winged insect
Could this be a fairy? Or could it be the fabled honey bee? Lorem ipsum etc
With appropriate alt-text, it’s still screen-readable, and you could play around with borders/fonts etc. so the sidebars look nice but don’t intrude too much on the main text.