Broadcast domain vs. Collision domain
The broadcast domain is a domain which is a kind of a fragment or a division of computer networking in which all the nodes can be broadcasted through the data link.
The broadcast domain can work on one network or (through a bridge) can work on a different LAN network.
There is only one single broadcast domain which connects all of the devices and this happens as a consequence to the fact that the devices that is being used in the first place, through accessing data sends out a broadcast to all the other devices and they get a copy of it.
So in that case, what is a collision domain?
A collision domain can only be used in certain ways, meaning that only one device can be used at a time. If two devices are to be used simultaneously on a LAN network, then collision would occur and thus make the network ineffective.
That is why modern networking uses switches to evade these collisions and their reoccurrence.