Penetration aid

A penetration aid (or "penaid") is a device or tactic used to increase an intercontinental ballistic missile (ICBM) warhead's chances of penetrating a target's defenses.

These can consist of both physical devices carried within the ICBM (as part of its payload), as well as tactics that accompany its launch or flight path, and may include one or more of the following (and more) concepts :

Carrying such devices has a price in terms of payload weight and volume, which requires a compromise versus warhead size and numbers onboard, as well as missile range.

...
Wikipedia