- Mandatory evacuation
- An official order to leave an area. Comply: afterward there may be no rescue. See ch. 24.
- MAYDAY
- A distress call by radio, repeated three times, when life is in danger on the water or in the air. On boats, VHF channel 16 is used. See ch. 13.
- MERV 13
- A rating for air filters that capture fine particles such as smoke well. The basis of the homemade Corsi-Rosenthal box. See ch. 18.
- Meshtastic
- An open-source project with small LoRa radios that form a text-messaging network with no cell service or internet; each radio relays the messages. See ch. 19.
- Mil (2–4 mil plastic)
- A measure of plastic thickness (thousandths of an inch). For sealing windows and vents, 2 to 4 mil is used. See chs. 16 and 18.
- Morse code (SOS)
- A code of dots and dashes. SOS is ··· ––– ···, with light, whistle, or knocking. See ch. 8.
- MRE (Meal, Ready-to-Eat)
- A ready-to-eat military ration. It lasts years in a cool place, but much less in heat. See ch. 22.
- Mylar bag
- An aluminized polyester bag that blocks light, moisture, and oxygen. With an oxygen absorber, it is used to store grains for years. See ch. 22.
