A collision domain:
It is an area in a network where two ethernet frames can collide on one link between two devices. If one emits and if the other emits data at the same time, it creates the collision.
It only happened in early version of ethernets when devices where half duplex, way back! At that time the switch were called HUB. If i remember well, they used to repeat the electric signal & forward it to all other ports. They didn't have an ARP table with the list of MAC addresses from every devices connected to the hub.
Hubs were Layer 1 devices.
Sometimes devices send ethernet frames to a mac address destination: FF:FF:FF:FF:FF:FF, it means that it is for all devices that are directly connected to the emitting device. FF:FF:FF:FF:FF:FF is what i call the broadcast mac address. All the devices who receive it are connected by a SWITCH or switches.
Nevertheless, a broadcast domain stops at the router because your router job is to remove the header of the frame including the destination MAC address with another destination MAC address to send it to another broadcast domain (another network which can be a public network).
Broadcast domain is on layer 2 (data link).