E[X|Y, Z] = E[X|Y]

I am given a problem with this condition. Please tell me what the left hand side term mean?? and what can we write for it??

E[X|Y, Z] = ??

(I expect something in terms of Joint PDF or Conditional PDF.If that cannot be done, please tell me any other way)

I know what to write for E[X|Y]. But what should we write for E[X|Y, Z]?? URGENT!! (Tell me if my question is not clear!!)