Answer:
A unit of area exactly equal to 4840 square yards or 43560 square feet.
4840 seems like a strange number, but it makes sense in the imperial measurement system. To measure lengths in the imperial system, 22 yards makes a chain, otherwise known as 'the length of a cricket pitch'. Ten chains makes a furlong (as seen in horse racing), and 8 furlongs make a mile. An acre is an area one chain by one furlong (22 x 220 yards). This is a convenient measure of area if you are planning crops on a small farm! (I find it easier to visualize an area 2 cricket pitches by 5 cricket pitches).
Converting into metric doesn't give an exact number, but an acre is approximately
4046.86 square metres. Football pitches are not a standard size, but a typical football pitch would be about 1 acre, perhaps a little more. WHAT KIND OF FOOTBALL PITCH?! The metric equivalent is the hectare, which is exactly 10,000 square metres - so one hectare is about 2.5 acres.
Acre is a measurement of area, so it does not necessarily have a given length or shape. If it was square shaped an acre would be 208.71 feet long on each side.
From: 1 acre = 43,560 square feet. Take the square root of 43,560 to get 208.71.