The nuclear triad refers to the three ways in which nuclear weapons can be delivered. The three ways in which these weapons can be delivered from one country to another are 1) by airplane, 2) by missiles based on the ground, and 3) by missiles launched from submarines. These are the three parts of the nuclear triad.
What this means is that any weapons systems that are used to deliver nuclear weapons in any of these ways are parts of the nuclear triad.
These weapons systems have different names in different countries. For example, the United States’ submarine-based nuclear missiles are called Trident missiles. The Russians also have missiles that launch from submarines. Their missiles are called Bulava. These are both parts of the nuclear triad, but they have different names.
The weapons systems that are part of the nuclear triad, then, are any nuclear weapons that can be delivered from airplanes, from the ground, and from submarines.