To encrypt something is to turn it into a special code so that strangers who see it cannot read it. Only someone with the right key — a password, a secret number, a matching program — can turn it back into ordinary words. We do this most of all with computers: messages, bank details, photos on a phone. Though actually, the idea is older than computers; armies and lovers have long hidden writing. Older people may still hear a spy story in the word; younger people meet it every day in apps that promise a private chat. It is a fairly computer-ish word that has walked into ordinary life because so much of life is on a screen. Nearby word, different job: encode can mean “put into any kind of symbols,” even ones that are not secret. Encrypt is specifically to hide.