The phrase "temporary expression" does not seem to be a defined term. It is used in the following contexts:

[expr.comma]/1:

If the right operand is a temporary expression ([class.temporary]), the result is a temporary expression.

[class.base.init]/8:

A temporary expression bound to a reference member in a mem-initializer is ill-formed.

[class.base.init]/11:

A temporary expression bound to a reference member from a default member initializer is ill-formed.

The sentence in [expr.comma]/1 was added by CWG 462 and has editorial adjustments applied later. CWG 462 should probably be superseded by CWG 1299 and thus that sentence could be removed.

The ones in [class.base.init] could probably say "temporary object" rather than "temporary expression".