A quincunx is a set of five objects, arranged so that four are at the corners of a square or rectangle and the fifth is at its centre. Take a look at the five on a dice (a die if you’re American), or the five of a suit of cards. In each case the dots or pips are arranged in this distinctive shape.
The word comes to us from Latin, in which it literally means “five twelfths”, from quinque, five, plus uncia, a twelfth. The latter word, by the way, is also the source of our inch and of ounce (there are sixteen ounces to the pound that is used in some countries today, but that’s a medieval innovation — the troy pound employed for precious metals and gems keeps the older twelve ounces). The Romans used quincunx as a symbol or marker for five-twelfths of an as, the latter being a Roman copper coin which at one time weighed twelve ounces (which could be classed as an item of small change only if you are halfway to being a giant).
Learned Englishmen brought it into the language in the seventeenth century to refer to things arranged in this characteristic way. An early user was Sir Thomas Browne, in his Garden of Cyrus of 1658; this is a work of fantasy in which he traces the history of horticulture down to the time of the Persian King Cyrus. The king is credited with having been the first to plant trees in a quincunx, though Browne claimed to have discovered that it also appeared in the hanging gardens of Babylon. The diarist John Evelyn soon followed Sir Thomas’s lead — in his book on orcharding, Pomona, he suggested it was a convenient way to lay out apple or pear trees. At about the same period, quincunx began to be used in astrology to refer to an aspect of planets that are five signs of the zodiac apart (out of the twelve).
If you need the adjective (although hardly anyone ever does), it’s quincuncial.