A guide is a person who leads or directs another person over unknown or unmapped country, or conducts travellers and tourists through a town or other place of interest.
A tour guide is a person who leads tourists or other travelers around a town, museum, or other tourist site, or on a longer tour along a fairly well established tourist circuit. Such a tour is called a "guided tour". When the guide works at a particular location, such as a museum, they may be called a docent. Or they may lead an individual or group as part of a package holiday.
A psychedelic guide is someone who takes an active role in guiding a drug user's experiences as opposed to a sitter who merely remains present, ready to discourage bad trips and handle emergencies but not otherwise getting involved. Guides are more common amongst spiritual users of entheogens. Psychedelic guides were strongly encouraged by Timothy Leary and the other authors of A Guide Based on the Tibetan Book of the Dead. Trip sitters are also mentioned in the Responsible Drug User's Oath.
But the necessity for such precautions died away when adequate surveys (in which guide officers were, at any rate in Kingdom of Prussia, freely employed) became available, and, as a definite term of military organization to-day, “guide” possesses no more essential peculiarity than "fusilier", "grenadier" or "rifleman". The genesis of the modern “ Guides” regiments is perhaps to be found in a short-lived Corps of Guides formed by Napoleon in Italy in 1796, which appears to have been a personal escort or body guard composed of men who knew the country.
In the Belgian army the two Guides regiments constituted part of the light cavalry. Until the outbreak of World War I these units were characterised by their green, yellow and crimson uniforms plus the generally aristocratic antecedents of their officers. As such the Belgian Guides came to correspond to the Guard cavalry of other nations. They served with panache (and still in green and crimson) during the German invasion of August 1914. The Guides exist today as an amoured regiment.
In the Swiss army prior to 1914 the squadrons of blue uniformed “Guides” acted as divisional cavalry. In this role these light cavalry units would have been called upon, on occasion, to lead columns. They were distinct from the green coated Dragoon Regiments who made up the line cavalry.
The “Queen’s own Corps of Guides” of the Indian army consisted of a unique combination of infantry companies and cavalry squadrons. After World War I the infantry element was incorporated in the 12th Frontier Force Regiment and the Guides Cavalry formed a separate regiment, which is now part of the Army of Pakistan. The Corps of Guides were the first military force to adopt khaki as a service dress, in 1849.
In drill, a “guide “ is an officer or non-commissioned officer told off to regulate the direction and pace of movements, the remainder of the unit maintaining their alignment and distances by him.
In mechanical usage, the term "guide" has widespread applications, being used of anything which steadies or directs the motion of an object, as of the “leading” screw of a screw-cutting lathe, of a loose pulley used to steady a driving-belt, or of the bars or rods in a steam-engine which keep the sliding blocks moving in a straight line. The doublet “guy“ is thus used of a rope which steadies a sail when it is being raised or lowered, or of a rope, chain or stay supporting an object such as a funnel, mast, derrick or tent.
In the Indian Academia the word Guide is referred to the person who helps you during preparing a Doctorate or Ph.D. thesis.
Original text from the 1911 Encyclopædia Britannica.