Unevaluated

Unevaluated(expr)

temporarily leaves expr in an unevaluated form when it appears as a function argument.

Unevaluated is automatically removed when function arguments are evaluated:

>> Sqrt(Unevaluated(x))
Sqrt(x)

>> Length(Unevaluated(1+2+3+4))
4

Unevaluated has attribute HoldAllComplete:

>> Attributes(Unevaluated)
{HoldAllComplete,Protected}

Unevaluated is maintained for arguments to non-executed functions:

>> f(Unevaluated(x))
f(Unevaluated(x))

Likewise, its kept in flattened arguments and sequences:

>> Attributes(f) = {Flat};

>> f(a, Unevaluated(f(b, c)))
f(a,Unevaluated(b),Unevaluated(c))

>> g(a, Sequence(Unevaluated(b),Unevaluated(c)))

However, unevaluated sequences are kept:

>> g[Unevaluated[Sequence[a, b, c]]]
g(a,Unevaluated(b),Unevaluated(c))