A sextant is a device for measuring the celestial altitude of a celestial body.
It consists of a telescope and a system of adjustable reflectors for presenting the image of the celestial body to the eye so as to be next to an image of the horizon.
The resulting reading on the scale of the sextant gives an indication of the altitude of the body.
The principle behind the operation of the sextant was invented by Isaac Newton, and can be found in his unpublished writings.
The technique was reinvented (and implemented) independently by both Thomas Godfrey and John Hadley in around $1730$.
Its main purpose in the context of navigation was to determine (terrestrial) latitude, by measuring the altitude of a particular celestial body, usually Polaris or the Sun at noon.
The advent of GPS has rendered use of the sextant more or less obsolete.
The word sextant derives from its shape, which is $\frac 1 6$ of a circle, from the Latin sextans, meaning one sixth.