What Are Securities?
The Short Explanation:
The definition of securities tend to have a different meaning all around the world. In the US typically all financial assets that can be traded can be considered as securities. The FCA defines securities differently in the UK. Most common types of securities are Equity Securities (such as stocks) Debt Securities (such as bonds) and Derivatives (such as options and futures).
For the most up-to-date definition that applies to the UK only:
See the full definition of a security by the FCA here.