The alternate key is a secondary key.

Let us take an example table

1NithinBank StreetHyderabad
2BharathWhite FieldBangalore
3GouthamCourt roadChennai


Here Student ID is Primary key and rest of all columns is defined as Alternate keys.

In general Alternate key can be defined as the key which is not a primary key.

Here in the table Student_Name, Address, City are called as Alternate Keys.