Correct spelling, explanation: to differentiate between on board and onboard, you can consider the context of the sentence. If you are discussing physical presence or inclusion within a group or vehicle, use on board (two words). If you are describing built-in functionality or components within a system or device, use onboard (one word). As an adjective, onboard always comes before the noun it modifies. After a verb, choose the phrase on board instead.

Definition of onboard:
carried within or occurring aboard a vehicle
The onboard computer system is responsible for controlling various functions of the spacecraft during the mission.
The airline offers onboard entertainment, allowing passengers to enjoy movies and games during their flight.

Collocations with onboard:
Some most commonly used collocations include:
1. Onboard equipment: refers to the various tools, devices, or machinery integrated into a vehicle, system, or equipment.
The aircraft’s onboard equipment includes radar, navigation systems, and communication devices.
2. Onboard systems: describes the integrated components and mechanisms within a particular system or machine.
The car’s onboard systems ensure safety and efficiency, including the anti-lock brakes and airbags.
3. Onboard technology: refers to the advanced electronic and digital features incorporated into a device or vehicle.
The smartphone’s onboard technology allows users to access the internet, take high-quality photos, and run various applications.
4. Onboard services: denotes the amenities, assistance, or offerings available to passengers or users while inside a vehicle or facility.
The cruise ship offers a range of onboard services, including dining options, spa treatments, and entertainment.

Correct spelling, explanation: the phrase on board has its origins in nautical and maritime terminology, dating back to the early days of seafaring. It was originally used to describe the physical presence of individuals or cargo on a ship or vessel. Over time, it made its way into the general language to describe physical presence or inclusion within a group or organization, not just in a maritime context. Remember that on board and onboard have different meanings and usages. The first one deals with physical presence or inclusion, while the second describes the integration or inclusion of features within a system or device.