Sexually explocit pictures, writing, or other material of people who are between birth and puberty, in which the primary purpose is to cause sexual arousal.
Pictures of a 12 year old child having sex with another child/adult or masturbating would be considered Child Pornography.
Pictures of a 12 year old child naked in a non-sexual stance, wouldn't be considered Child Pornography.
Pictures of a 14 year old having sex with another person the same age or above, would NOT be considered child pornography by definition. Mainly because a 14 year old, is not a child.