Security refers to all the measures that are taken to protect a place, or to ensure that only people with permission enter it or leave it.

A feeling of security is a feeling of being safe and free from worry.

If something is security for a loan, you promise to give that thing to the person who lends you money, if you fail to pay the money back.

Securities are stocks, shares, bonds, or other certificates that you buy in order to earn regular interest from them or to sell them later for a profit.