Amendment II

A well regulated militia, being necessary to the security of a free state, the right of the people to keep and bear arms, shall not be infringed.

From reference 2 below:

(a) The Amendment's prefatory clause announces a purpose, but does not limit or expand the scope of the second part, the operative clause. The operative clause's text and history demonstrate that it connotes an individual right to keep and bear arms. Pp. 2-22.

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Definitions:

Definition of militia

1

a : a part of the organized armed forces of a country liable to call only in emergency The militia was called to quell the riot.

b : a body of citizens organized for military service

2 : the whole body of able-bodied male citizens declared by law as being subject to call to military service

Meriam-Webster

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References:
Legal Information Institute - U.S. Constitution Second Amendment
SUPREME COURT OF THE UNITED STATES DISTRICT OF COLUMBIA et al. v. HELLER