Alternative spelling of don't cry over spilt milk

You shouldn't be upset over something that can't be undone.

Alternative form of there's no point crying over spilt milk

It is unwise to be complacently happy because things may get worse.

The discipline concerned with communication security (eg, confidentiality of messages, integrity of messages, sender authentication, non-repudiation of messages, and many other related issues), regardless of the used medium such as pencil and paper or computers.

The subfield of cryptography in which all information used to encode a message is assumed to be publicly available.

(countable) A solid composed of an array of atoms or molecules possessing long-range order and arranged in a pattern which is periodic in three dimensions.

Very clear.

(optics) A doubly refracting crystal in which the index of refraction for the extraordinary ray is greater than for the ordinary ray, and the former is refracted nearer to the axis than the latter, such as quartz and ice; as opposed to negative crystal: one in which this characteristic is reversed, such as Iceland spar or tourmaline.

The cryopreservation of a person with medical needs that cannot be met by available medicine until resuscitation and healing by future medicine is possible.