Is a trap a holding tank for solid waste or the curved section of a drain

It's there to block sewer gas...

It is the curved part of a drain -called a P trap. Water in the trap keeps gaseous waste from entering your house and potentially causing harm to humans. That said, there are traps for solid waste, mostly found in restaurants and noted as grease traps. Years ago and in some third world countries, solid waste was piped outside and "trapped" in tanks that were/are them pumped out but there had to be a open spot so gasses can not come back. What prompted the question?

There are many types of traps. They all contain water to prevent sewer gas from entering your home.

Is a trap a holding tank for solid waste of this is called a septic tank.
A trap is curvy pipe under a drain. Cheers, Tom