Ex- is a prefix that is used before a noun. It means ‘former’:

ex-president
(he was the president, but now he is not)

ex-policeman
(former policeman)

ex-member

ex-prisoner

 

Ex can also be used as a noun by itself. An ex refers to former romantic partner:

ex-girlfriend

ex-boyfriend

ex-wife

ex-husband

 

If someone says ‘my ex,’ you’ll know whether they mean ex-girlfriend/boyfriend/wife/husband if you know their personal history or from the conversation.

My ex has a new girlfriend. She’s ugly.

He and his ex went through a bitter divorce but they are getting along better these days.

Do you still keep in touch with your ex?