The term security in legal English has two important but very different meanings.
A security (plural: securities) is an investment instrument issued by a company, government, or other organization which offers evidence of debt or equity, or to investments such as shares or bonds.
“The legislation prohibits trading by brokers in certain types of securities.”
The term security (note that no article is used) refers to the provision of rights against a person’s assets which are intended to enhance the probability of recovering a claim against him.
“A mortgage was taken by the bank over the property as security for the loan.”