Significant Other

A significant other is what you someone you are in a relationship with without disclosing gender identity, relationship/marital status, or sexual orientation.

Commonly used to describe someone who is non-binary/genderqueer that you or someone else is in a relationship with, as well as with people who don't want people to know their sexual orientation (bisexual, pansexual, etc).
Though, it can have different meanings depending on how you use it. Such as describing someone you're dating/not in a relationship with yet.

Example 1:

"Jessa recently told me that she is in a relationship with someone!"
"Really? What's her boyfriend's name?"
"Actually, they are nonbinary/genderqueer so they prefer 'significant other'."

Example 2:

"I'm going on a date tonight with my significant other!"
"Awesome! Is it a boy?"
"It's none of your business."

Example 3:

"How is it going with you and Alexa?"
"At the moment, we're not making it official so she's just a significant other."