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Innocence is the opposite of guilt. The adjective is innocent (the opposite of 'guilty').
If someone is innocent of a crime it means that they did not do it. If someone is accused of a crime, they have to go to court. The court will decide whether they are guilty or not guilty (innocent). People must be treated as innocent unless (or until) they are found guilty.
'Innocence' and 'innocent' can also be used in a slightly different way. Children are often described as 'innocent' meaning that they think the world is all good and do not yet know about the bad things in life. People talk about living in a 'state of innocence' i.e. children or people with simple minds who have not yet learned that some people are evil. In Christian cultures this sometimes meant that children did not know about sex.