A person who hasn't made it past 2nd base. Someone who has never been touched in a sexual way over or under clothing and hasn't touched the other gender's genitals, includes having sex.
David: Bro, I took Layla's innocence last night when she gave me a handjob.
Andy: Awesome! It's about time, she was innocent for way too long!
To touch a woman's breast or Pussy, or a Man's Penis or Balls without having sex with them therefor not ruining their virginity but their innocence instead.