Rest (Old English ræst, reste, bed, cognate with other Teutonic forms, e.g. German Rast, Rilste, rest, and probably Gothic Rasia, league, i.e. resting or stopping place), is a cessation from active or regular work, hence a time of relief from mental or manual labour. Specific meanings are for an interval of silence in Music, marked by a sign indicating the length of the pause; for the forked support with iron-shod spike carried by the soldier till the end of the 17th century as a rest for the heavy musket; and for the support for the cue in billiards to be used when the striking ball is out of reach of the natural rest formed by the hand.
In the medieval armour of the horsed man-at-arms, and later in the armour of the tournament, a contrivance was fixed to the side of the body-armour near the right arm-pit, in which the butt-end of the lance was placed to prevent the lance being driven back after striking the opponent at full charge. Hence a knight, as a preliminary to the charge, laid his lance in rest. This rest is a shortened form of arrest, to check, stop, as is seen by the French equivalent, arret.
In Physics and in the technical sense of geometric mensuration, rest denotes a particular relation between a pair of observers. By Albert Einstein's celebrated definition, two observers measure having been at rest to each other in any particular trial if they succeed to identify a third observer as middle between each other, in that trial.
Further, rest, that which remains over and above, is derived from the French rester, to remain over, Latin restare, to remain, literally, to stay behind. The principal specific use of this word is in commerce for the balance of undivided profit; it has thus always been the term used by the Bank of England for that which in other banks and companies is called the reserve.
