Is there a difference between PCB trace impedance and transmission line characteristic impedance?
So is the PCB trace impedance an impedance or a resistance?

It's both (short story)...
The difference between a cable and a printed circuit board track is length. Cables can be miles long but a PCB trace is likely to be no longer than a foot. At a foot length (300 mm), a signal frequency having this wavelength is about 1 GHz. However, we say in EE that anything approaching one tenth of a wavelength is starting to be significant hence, for a foot (300 mm) anything around 100 MHz or above is relevant to start thinking about characteristic impedance and, importantly anything seriously higher than 1 MHz is going to have a purely resistive impedance.
The general formula for a t-line is this: -
$$Z_0 = \sqrt{\dfrac{R + j\omega L}{G + j\omega C}}$$

R is the series loop resistance per metre
L is the series loop inductance per metre
G is the parallel conductance per metre
C is the parallel capacitance per metre

As frequency rises beyond several hundred kHz, the \$j\omega\$ terms dominate and we get this: -
$$Z_0 = \sqrt{\dfrac{j\omega L}{j\omega C}} = \sqrt{\dfrac{L}{C}}$$
That formula is not related to frequency and it is also resistive.
Finishing off; any PCB t-line calculation won't bother thinking of anything other than resistive terms because it just won't be physically long enough to be a practical consideration.