title: Foreign keys vs secondary keys
body: <p>I used to think that foreign key and secondary key are the same thing.</p>
  
  <p>After Googling the result are even more confusing, some consider them to be the same, others said that a secondary key is an index that doesn't have to be unique, and allows faster access to data than with the primary key.</p>
  
  <p>Can someone explain the difference?<br />
  Or is it indeed a case of mixed terminology?<br />
  Does it maybe differ per database type?</p>