A security is a fungible, negotiable financial instrument that represents some type of financial value. The company or entity that issues the security is known as the issuer.

In foreign exchange markets (Forex), a security is also called a 'pair' of two assets (currencies): EUR/USD, GBP/USD, ...

In equities markets a security can be also considered a 'pair' between a company stock and a currency: ITT_Corporation/USD, Santander/EUR, ...

The first element on each pair is known as the 'base' asset. The last element on each pair is known as the 'term' asset.

Buyers and sellers trade with securities in different markets. When a buyer 'buys a security', the buyers buys the 'base' asset and sells the 'term' asset. When a seller 'sells a security', the seller sells the 'base' asset and buys the 'term' asset.

EUR/USD : Base asset is 'EUR' and term asset is 'USD'.