Relationships

My boyfriend called me his "other half".. what does this mean?

Me and my boyfriend been together for over a year
We were hanging out and he wanted to take pictures of us both
So we took photos together
And then he uploaded one of the pictures on his facebook and instagram
And he put the caption "spending quality time with my other half 😊"
I was like in my head 'wow he called me his other half?'
I thought that happens when people are married and they say that to each other
What does "my other half" even mean?
My boyfriend called me his "other half".. what does this mean?
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