adj

• (euphemistic, preceded by "a") Used to denote that the speaker is referring to a specific person or thing that they do not want to name directly, implying that the listener should infer the identity of the referent.
  "I would have been here on time, but a certain someone lost the car keys!"
• (preceded by "a", of a person) Named but not previously mentioned.
  "Looking inside the cover, they learned that the book had once belonged to a certain R. Jones."
• (preceded by "a", of a person) Used before the name of someone famous that people are expected to know.
  "Since the last British government to make such a proposal was that of a certain Margaret Thatcher, it might not seem unreasonable."
• (obsolete) Determined; resolved.
  "However I with thee have fixt my Lot, Certain to undergoe like doom[…]"

pron

• (with of) Unnamed or undescribed members (of).
  "She mentioned a series of contracts, of which certain are not cited."