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!
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.