Doesnt resistance mean same as impedance??

İmpedance is resistance which changes with frequency whereas simple resistance is constant with frequency. 

"There is NO frequency with DC, which is WHY, "resistance is constant".

The above is NOT true. You can still apply AC to simple resistance and its resistance will still NOT change, whereas a capacitor and/or an inductor’s resistance WILL change with AC.

Here is a table which shows how various components will respond to application of AC or DC voltage:

                  Resistor                            Inductor                    Capacitor
-----------------------------------------------------------------------------------------------

DC             constant R                        transient                   transient

AC             constant R                        varying R                  varying R