Be an item
Definition 1 of 2
Pro Tip 1/3Informal Usage
Used mainly in casual conversations, not in formal writing. 
Did you hear? Mia and Alex are an item now.
Pro Tip 2/3Gossip Context
Often used when discussing or speculating about others' relationships. 
I saw Josh and Emma together at the cinema. Are they an item?
Pro Tip 3/3Current Relationship
Implies that the relationship is happening now. 
Lara and Sam are an item, they just celebrated their one-month anniversary.