The alternate key is a secondary key.

Let us take an example table

StudentIDStudent_NameAddressCity
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.