they

A group of entities previously mentioned.

• Fred and Jane? They just arrived.
• Dogs may bark if they want to be fed.
• Plants wilt if they are not watered.

A single person, previously mentioned, whose gender is unknown, irrelevant, or non-binary. One whose gender is unknown, irrelevant, or both.

• Somebody requested a seat at Friday's performance but didn't say if they preferred the balcony or the floor. [The gender is unknown and irrelevant.]
• I asked my friend if they wanted to come, but they said no. [The gender is known but irrelevant.]
• One of the boys tripped over, and they hit their head on the door. [The gender is known but irrelevant; the particular subclass of usage in this example was seldom accepted before the 21st century, but its acceptance is increasing.]

One whose gender is unknown, irrelevant, or both.

• Somebody requested a seat at Friday's performance but didn't say if they preferred the balcony or the floor. [The gender is unknown and irrelevant.]
• I asked my friend if they wanted to come, but they said no. [The gender is known but irrelevant.]
• One of the boys tripped over, and they hit their head on the door. [The gender is known but irrelevant; the particular subclass of usage in this example was seldom accepted before the 21st century, but its acceptance is increasing.]

One who is nonbinary.

• There is no need to panic. Robin should be here soon and they will know what to do.

People; some people; people in general; someone, excluding the speaker.

• They say it’s a good place to live.
• They didn’t have computers in the old days.
• They built this city on rock and roll.

The authorities, the (power) elites, the powers that be, the establishment, the man, the system: government, police, employers, etc.

• They'll tax us for the air we breathe next.
• They should increase our wages.
• Ha, you believe the moon is real? That's just what they want you to think.

The opponents of the side which is keeping score.

The, those.

Their.

To refer to (someone, sometimes especially someone who does not use gender-neutral pronouns) using they/them pronouns.

• I have a pin that says she/her, but I still get theyed all the time.

A non-binary person.

There .