# How to prevent substitution?

I have a function:

$\bar{u}:=u[t,x,y,z]$ (1)

How do I prevent it from expanding and substituting in instructions like those following:

$\partial _t {\bar u}$ (2)

OverBar[u]:=u[t,x,y,z]

D[ OverBar[u] , t]

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Please include Mathematica code, not latex expressions. Have you looked at Hold, HoldForm and related functions or Defer? – R. M. Sep 14 '12 at 7:39
I think this question needs more context. – Mr.Wizard Sep 14 '12 at 7:50

If you would like to display the expression without evaluation use HoldForm
HoldForm@D[OverBar[u], t]