In modern usage, a sloop is a sailboat with a single mast^[2] generally having only one headsail in front of the mast and one mainsail abaft (behind) the mast. It is a type of fore-and-aft rig. The mainsail may be of any type, most often Bermuda rig, but also others, such as gaff or gunter.

In naval terminology, "sloop-of-war" refers to the purpose of the craft, rather than to the specific size or sail-plan, and thus a sloop should not be confused with a sloop-of-war. As with many rig definitions, it was some time before the term sloop referred to the type of rig.

Regionally, the definition also takes into account the position of the mast. A forward mast placement and a fixed (as opposed to running) bowsprit, but with two headsails may give categorisation as a sloop. An example is the Friendship Sloop.^[1]: 48-53

The name originates from the Dutch sloep, which is related to the Old English slūpan, to glide.^[3] The original Dutch term applied to an open rowing boat.^[1]: 49 A sloop is usually regarded as a single-masted rig with a single headsail and a fore-and-aft mainsail. In this form, the sloop is the commonest of all sailing rigs – with the Bermuda sloop being the default rig for leisure craft, being used on types that range from simple cruising dinghies to large racing yachts with high-tech sail fabrics and large powerful winches.^{[1]: 48–53}If the vessel has two or more headsails, the term cutter is usually applied, though there are regional and historic variations on this. A boat with a forward mast placement and a fixed bowsprit, but more than one headsail, may be called a sloop. The Friendship sloop is an example of this. Particularly with historic craft, categorisation as a cutter may rely on having a running bowsprit.^{[a][1]: 49–55}

Before the Bermuda rig became popular outside of Bermuda in the early 20th century, a (non-Bermudian) sloop might carry one or more square-rigged topsails, hung from a topsail yard and sheeted to a lower yard.^[4]

A sloop's headsail may be masthead-rigged or fractional-rigged. On a masthead-rigged sloop, the forestay (on which the headsail is carried) attaches at the top of the mast. On a fractional-rigged sloop, the forestay attaches to the mast at a point below the top. A sloop may use a bowsprit, a spar that projects forward from the bow.

• 
  Bermuda-rigged sloop. The jib is a headsail. See cutter rig for other examples of headsails.
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  Gunter-rigged sloop
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  Gaff-rigged sloop with a headsail and a gaff topsail
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  Spritsail sloop
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  Sloop sail with multiple foresails and topsail

• Bermuda Fitted Dinghy: a scaled-down sloop used for racing in Bermuda
• Bermuda sloop, originally used for a type of sea-going, sloop-rigged vessel. Today used for any Bermuda-rigged sloop
• Chialoup, a historical type of sloop produced in the East Indies
• Mast aft rig, a single mast rig with a mast further back than a sloop or cutter

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