Definition
An individual or object that serves no useful purpose.
A third party, usually unwanted, accompanying two people on a date.
How People Actually Use It
When you are hanging out with two other people usually friends, siblings, or relatives. They always talk to each other and have fun, but you are just tagging along and overhearing what they say. They act like you are not there. Sometimes you have no one else to hang out with so you just go with them. You will always feel left out.
"Someone-“I feel bad for your sister she is always the third wheel”"
Source: Urban Dictionary