Fallout 4's Survival Mode is what most people here are using as a reference. It includes these features:
* Manual saving, quicksaving, and autosaving (with the exception of autosaving after sleeping) are disabled. The only way to save is to sleep in any bed for at least one hour.
* Fast travel is disabled.
* The Sole Survivor takes but also deals more damage.
* Most threats are not visible on the compass.
* Locations of interest are harder to detect.
* A new perk, Adrenaline, is automatically given to the Sole Survivor.
* The Sole Survivor must stay hydrated, eat periodically, and have a regular sleep cycle in order to survive.
* Fatigue damage reduces Action points (AP).
* Sleeping in a bed for less than 7 hours does not heal Hit points (HP).
* Illness and antibiotics are introduced.
* The healing rates of RadAway, stimpaks, and other healing items are significantly reduced.
* RadAway and chems can negatively affect the immune system.
* Limbs no longer heal after combat and must be restored with a stimpak, sleeping in a bed until well rested, or by visiting a doctor.
* Ammunition has weight and total carry weight is significantly reduced for both the Sole Survivor and their companions.
* Companions will not get back up if downed in combat until healed. If left unattended, they will return home.
* Robotic companions created in the Automatron add-on, excluding NPC robots like Ada and Jezebel, will now be destroyed if not repaired after being downed in combat.
* Cleared locations (both those marked as "[Cleared]" or otherwise) take much longer to repopulate enemies and loot.
* Normally non-"[cleared]" locations take 7 in-game days and "[cleared]" locations take 20 in-game days to start respawning. In survival mode, these are increased to 35 in-game days and 80 in-game days, respectively.
* Perhaps as a consequence of less respawned enemies, enemies now yield twice as much experience. (This change is not explicitly mentioned by the help notes for survival mode.)
* Enemies that usually spawn at higher levels will now spawn with regularity.
* Ability to access the game console (the tilde key) and use console commands is disabled. (PC only)