How do you name this alkene?

In nomenclature of an alkene, the parent chain is the LONGEST CHAIN INCLUDING THE C=C DOUBLE BOND, i.e. pent-1-ene (or 1-pentene). An ethyl group is attached to carbon-2. Hence, the IUPAC name is :
2-ethylpent-1-ene (British) or 2-ethyl-1-pentene (U.S.)

(Note that it is wrong to choose the longest saturated hydrocarbon chain excluding the C=C double bond as the parent chain.)

The longest chain is six carbons long, so its a hexane. The double-bonded carbon atom is called a methylene group, so altogether it's called 3-methylenehexane, according to IUPAC.

It is also commonly called 2-ethyl-1-pentene.