Saturation attack
Saturation attack is a military tactic in which the attacking side hopes to gain an advantage by overwhelming the defending side's technological, physical and mental ability to respond effectively. During the Cold War and after the conventional saturation missile attack against naval and land targets was and is a much feared eventuality.
Theoretical calculations
Taking the design of an anti-ship missile as a theoretical example, one can begin with the goal of creating a missile which will get through and destroy its target 100% of the time. However, as a designer one knows that with time an opponent will develop counter measures that means that your missile will at some later date not be 100% effective. So given that a missile will only ever be less than perfect why not design a missile that will get through to the target say only 70% or even only 50% of time, with a cost saving benefit on each missile produced. Launched singly a missile with a 50% chance of getting through to its target will only get to its target 50% of the time, but with a salvo two such missiles the chance of at least one missile getting through is 75%, with three missiles 87.5% etc., with the probability that at least one missile will get through to its target becoming closer to certainty with each missile fired. If each missile has a warhead capable of destroying the target, then no individual missile can be ignored by the target warship and counter-measures. Thus, SAMs and CIW resources will have to be expended to defend the target. To further overload a target's ability to defend itself, an attacker can attempt to launch multiple missiles from multiple directions using different guidance options.
The main subscribers to the concept of the saturation missile attack were the Soviet Union and its client states. The Komar class missile boat for example being designed and operated around the calculations that it would take 12 P-15 Termit missiles to destroy a single NATO destroyer. At a time when British destroyers were equipped with just four anti-ship missiles, Soviet ships were going to sea with anything up to 20 anti-ship missiles, with even destroyers fielding 8 large missiles. With the NATO allies having followed the route of individually smaller and lighter missiles, NATO warships had the appearance of being underarmed when compared to Soviet ships with their multiple missiles in large container/launch tube housings.
Bomber stream
The bomber stream was a tactic pioneered by the RAF to overwhelm German air defences during the Second World War. The tactic relied on routing a greater number of bombers through a defensive sector than the amount which Germans could generate interception sorties. Although it was a certainty that bombers would be lost, it was impossible for the defending fighters to destroy every bomber and stop the bombers gaining their objective.
During the Cold War British and American air defences revolved around plans to stop massed Soviet bomber streams, leading to the US fielding nuclear tipped Nike missiles, and at a later date the British equipping trainers as interceptors in order to have enough fighters.
Countering
The only surefire way of countering a saturation attack is to destroy launch vehicles before they can begin their attack. For example, during the Cold War, the primary target of saturation attacks from the Soviet Naval Aviation, were the United States Navy aircraft carriers. In response the United States adopted the doctrine of attempting to destroy the Soviet missile aircraft before they could launch their missiles, this led to the Douglas F6D Missileer, which would give rise to the Northrop Grumman E-2 Hawkeye and Grumman F-14 Tomcat/AIM-54 Phoenix pairing.
In naval warfare, the incorporation of stealth technology in surface combatants the general adoption of vertical launching systems, modern radar systems which can simultaneously scan track and engage multiple targets, and fire and forget close in defense missiles has decreased the utility of saturation attacks by unsophisticated anti-ship missiles.