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A navigational hazard or hazard to navigation has been defined in various, slightly different, ways:

• An obstruction, usually sunken, that presents sufficient danger to navigation so as to require expeditious, affirmative action such as marking, removal, or redefinition of a designated waterway to provide for navigational safety.^[1]
• Any obstacle encountered by a vessel in route posing risk or danger to the vessel, its contents or the environment.^[2]
• An obstruction determined to have a substantial adverse effect on the safety and efficient utilization of the navigable airspace.^[3]

Maritime hazards to navigation and airspace hazards to navigation.^[3]

Hazards may be permanent, or temporary, including seasonal, and fixed or mobile,^[4]

• Fog is temporary, but may occur frequently in some areas and seasons
• Icebergs are mobile and temporary, and also seasonal in some areas
• Some river channels are variable
• Some underwater obstructions are unidentified, others may be known.
• Both shipwrecks with a fixed position and floating derelicts and other flotsam can be hazards
• Seabed obstructions
• Mined international waterways

The risk associated with a hazard is aggravated when the position is uncertain, or the hazard is unmarked or obscured by poor visibility.^[4]

• Marine accidents can occur, which can cause loss of life and vessels, or delays of shipping, unreliable transport of people and goods, and environmental damage.^[4]

• Weather conditions such as high winds, icing, thunderstorms, wind shear and clear air turbulence, low visibility.^[5]

• Physical obstructions such as tall buildings, radio masts, cranes, wires, mountains, cliffs, power lines.^[5]

• Volcanic ash.^[5]

• Smoke and convection from wildfires.^[5]

• Human factors, such as fatigue, poor navigation, inattention, bad communication and aircrew error.^[5]

• Entering restricted airspace without proper authorisationand warning.^[6]

• Wildlife such as birds can be a hazard, particularly during takeoff and landing.^[5]

• Dysfunctional navigation systems such as radio and radar beacons, lights, etc.

When deciding whether a static hazard will be marked,the following factors may be considered:^[7]

• Location of the obstruction relative to the navigable channel and relative to other hazards^[7]
• Difficulty of navigation near the obstruction^[7]
• Depth of water over the hazard, and how much it is likely to vary^[7]
• Type of vessel traffic in the vicinity of the hazard, particularly draft, but also amount of traffic^[7]
• Physical characteristics of the hazard^[7]
• Probability that the hazard may move^[7]
• Weather conditions that are likely in the vicinity^[7]
• How long the hazard has existed in that location, and any history of accidents involving the hazard,^[7] and
• Whether the object is considered a hazard in terms of alternative legislation^[7]

An aid to navigation (ATON) is any device external to a vessel or aircraft specifically intended to assist navigators in determining their position or safe course, or to warn them of dangers or obstructions to navigation.

• Lighthouse – Structure designed to emit light to aid navigation
• Lightship – Ship that functions as a lighthouse
• Navigational buoy – Moored floating object intended to aid navigation
  • Emergency wreck buoy – Marker buoy warning of a wreck
  • Isolated danger mark – Sea mark indicating a danger spot
  • Safe water mark – Type of sea mark
  • Cardinal mark – Sea mark indicating where safe water is near to a hazard
  • Bell buoy – Audible navigational warning used in low visibility
• Radar beacon – Transmitter-receiver associated with a fixed navigational mark
• Foghorn – Device using sound to warn shipping in fog
• Notice to mariners – Advisory document updating navigational safety information
• Nautical chart – Topographic map of a maritime area and adjacent coastal regions

A navigational warning is information published or broadcast providing information on the status of one or more navigational hazards.^[8]