The term "guide" refers to an agency for directing or showing the way, specifically a person who leads or directs a stranger over unknown or unmapped country, or conducts travellers and tourists through a town, or over buildings of interest.

Table of contents
1 Etymology
2 Military Usages
3 Mountaineering
4 Other Usages

Etymology

The word guide (Middle English gyde, derives from the from the French guide; and ultimately from the earlier French form guie (English “guy”). The /d/ sound originates with the Italian form guida; the word probably ultimately derives from the Teutonic, having connections with the base seen in Old English witan (to know).

Military Usages

In European wars up to the time of the French Revolution, the absence of large-scale detailed maps made local guides almost essential to the direction of military operations, and in the 18th century the general tendency to the stricter organization of military resources led in various countries to the special training of guide officers (called Feldjäger, and considered as general staff officers in the Prussian army), who had the primary duty of finding, and if necessary establishing, routes across country for those parts of the army that had to move parallel to the main road and as nearly as possible at deploying interval from each other, for in those days armies rarely spread out so far as to have the use of two or more made roads.

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 “ Guide” 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 Guide regiments came to correspond almost to the Guard cavalry of other nations; in the Swiss army the squadrons of “Guides” act as divisional cavalry, and in this role doubtless are called upon on occasion to lead columns. The “Queen’s own Corps of Guides” of the Indian army consisted of infantry companies and cavalry squadrons.

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.

Mountaineering

A particular class of guides are those employed in mountaineering; these are not merely to show the way but stand in the position of professional climbers with an expert knowledge of rock and snowcraft, which they impart to the amateur, at the same time assuring the safety of the climbing party in dangerous expeditions. This professional class of guides arose in the middle of the 19th century when Alpine climbing became recognized as a sport. It is thus natural to find that the Alpine guides have been requisitioned for mountaineering expeditions all over the world. In climbing in Switzerland, the central committee of the Swiss Alpine Club issues a guides’ tariff which fixes the charges for guides and porters; there are three sections, for the Valais and Vaudois Alps, for the Bernese Oberland, and for central and eastern Switzerland. The names of many of the great guides have become historical. In Chamonix a statue has been raised to Jacques Balmat, who was the first to climb Mont Blanc in 1786. Of the more famous guides since the beginning of Alpine climbing may be mentioned Auguste Balmat, Michel Cros, Maquignay, J. A. Carrel, who went with E. Whymper to the Andes, the brothers Lauener, Christian Almer and Jakob and Melchior Anderegg.

Other Usages

The word "guide" can also refer to a book or document, in the sense of an elementary primer on some subject, or of one giving full information for travellers of a country, district or town.

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.

Original text from the 1911 Encyclopaedia Britannica.

For the concept of a Guide in the Scouting movement see Guides