Answer:
Reagan's Strategic Defense Initiative (SDI), also known as "Star Wars," was a proposed missile defense system designed to shoot down incoming ballistic missiles in space. The idea behind SDI was to create a defensive shield that would render nuclear weapons "impotent and obsolete," and thereby make the concept of nuclear deterrence obsolete as well.
On the other hand, nuclear deterrence is a strategy of using the threat of nuclear retaliation to deter an adversary from attacking. The idea is that if a country knows that it will suffer devastating consequences in the event of a nuclear attack, it will be deterred from launching such an attack in the first place.
The key difference between SDI and nuclear deterrence is that SDI is a defensive strategy, while nuclear deterrence is an offensive strategy. SDI was designed to protect the United States from incoming ballistic missiles, whereas nuclear deterrence relies on the threat of offensive nuclear retaliation to prevent an attack from occurring in the first place.
Another key difference is that SDI was a departure from traditional arms control and disarmament efforts, which had focused on limiting the number of nuclear weapons in existence. SDI was seen by some as a way to undermine arms control efforts, and to potentially trigger a new arms race between the United States and the Soviet Union.
In summary, Reagan's Strategic Defense Initiative plan was different from the strategy known as nuclear deterrence in that it was a defensive strategy based on developing a missile defense system, while nuclear deterrence is an offensive strategy based on the threat of nuclear retaliation to prevent an attack.