Girlfriend
Definition 1 of 3
Pro Tip 1/3
Romantic Relationship
Use 'girlfriend' to describe someone you're dating or having a romantic relationship with, not just a friend. He's taking his girlfriend to the concert this Friday.
Pro Tip 2/3
Gender Specific
'Girlfriend' specifies the person is female, for a male partner, use 'boyfriend'. My sister said her girlfriend is a great cook.
Pro Tip 3/3
Not Only Sexual
'Girlfriend' can imply a serious or casual romantic relationship without emphasizing sexuality. They've been girlfriends for a couple of months now.