What is gender?

Recently got into a discussion with a friend on what my opinion of gender was. Surprisingly I found out that I couldn't really form an opinion becuase no one seems to agree on what the definition of gender is. It's been a huge topic of interest for me lately and I don't think I can solidly come up with a conclusion becuase both sides appear to have merit, however, it's difficult to debate becuase you begin to debate definition which is kind of pointless. I'd love to spark some conversation on the idea. So what is gender? Any opinions, ideas, and/or facts are welcome!