For example, would a laptop power adaptor with a voltage input of 100-240V ~ be safe to use in a country with 230V RMS wall voltage? Thanks in advance!
It's RMS voltage. In general, AC voltage means RMS voltage.
Mains distribution system voltages are always measured in VRMS unless otherwise specified.
Laptop power-supplies are switched-mode (SMPS) type and are designed to accept 100 - 240 V AC (RMS) to facilitate use on the common international voltages of 110/120 V and 230/240 V.
Internally the voltage is rectified to generate a high voltage DC. This is then switched at high frequency, and fed through an isolating transformer to the low-voltage rectifier and filter before feeding to the load. The switcher accommodates the variation in input voltage. Note that since P = VI that the input current will have to be higher when on a low voltage supply to generate the required power output.