In a game of cricket, a set of six consecutive deliveries (plus any wides and no-balls) bowled by one bowler from one end of the pitch to whichever of the two batsmen is at the other ("striker's") end; the standard unit of measurement of how long a team's innings is. (In the pre-war era eight ball overs were once a common alternative, particularly in Australia, but no idea where Webster got four from, even in 1913). The umpire standing at the bowler's end is responsible for keeping count of deliveries (and his count stands even if he makes a mistake; 5 and 7 ball overs happen from time to time even at the highest levels of the game); at the end of the over he calls "Over", and a new over is commenced by a different bowler from the other end of the pitch, with the umpire who was previously standing at square leg taking the position at the new bowler's end.
The standard pattern of play is for each bowler to bowl a spell of several alternating overs (say 3 to 12 - usually fewer for faster bowlers) from one end of the ground, and statistics and scorekeeping systems reflect this.
In a bizarre aberration from common sense, it is standard practice to refer to part-completed overs using a notation indistinguishable from a decimal point, whereby the part after the point is the number of balls in the incomplete over: "10.3 overs", often also enunciated as "ten point three overs" means ten overs and three (legal) deliveries of the eleventh.
"One-day" or limited overs cricket matches involve a fixed number of overs to be bowled by each team (if they don't bowl their opponents out beforehand); the norm for international matches is 50 overs per side. The more traditional forms of cricket (including test and first class matches) are limited by time rather than number of overs, but minimum numbers of overs per day, session or hour may be enforced by the addition of playing time and/or the levying of fines on teams. An over usually takes between 3 and 6 minutes to play, depending largely on how long the bowler's run-up is.