credential vs username

credential

noun
  • documentary or electronic evidence that a person has certain status or privileges 

verb
  • to furnish with credentials 

adj
  • Pertaining to or serving as an introduction or recommendation (to someone). 

username

noun
  • A person's identification on an individual computer system. 

How often have the words credential and username occurred in a corpus of books? (source: Google Ngram Viewer )