Moreover, you could have something like a Mamiya C330 which is a ILC, but not an SLR , does use a mirror for the finding lens but not for the taking lens .And then Sony comes and makes a camera that is non-reflex but has a mirror... Neither SLR no mirrorless... Argh!!!The camera trade organization CIPA puts fixed-lens compact cameras in a different category than mirrorless interchangeable-lens cameras.
Their official categories are:
Camera with built-in lens.
Camera with interchangeable lens: single lens reflex
Camera with interchangeable lens: non-reflex (meaning mirrorless)
***
https://en.m.wikipedia.org/wiki/Sony_SLT_camera
Any of the old large format cameras could arguably be mirrorless too .