security is protection against attack, danger, or something bad that might happen in the future, and also the state of feeling happy and safe from danger or worry.
Oxford Learner's Dictionaries records several practical senses. Security is the activities involved in protecting a country, building or person against attack or danger; the department of a large company or organization that deals with the protection of its buildings, equipment and staff; and, at an airport, the place a traveler goes after the passport check so that officials can find out whether they are carrying illegal drugs or weapons. In a broader sense it is protection against something bad that might happen in the future.
The dictionary also lists an emotional sense, the state of feeling happy and safe from danger or worry, and two financial ones. As a pledge, security is a valuable item, such as a house, that a borrower agrees to give to the lender if unable to pay back the money borrowed. In the plural and labeled finance, securities are documents proving that somebody is the owner of shares or similar holdings in a particular company.
The word dates from late Middle English, from Old French securite or Latin securitas, from securus, "free from care," built from se- ("without") and cura ("care").