Correct spelling, explanation: words on board and onboard are both correct, but they have different uses. On board is a phrase used when referring to someone or something physically present or included within a vehicle, vessel, team, organization, or group. It indicates that a person or object is inside or a part of something. Onboard is an adjective used to describe something that is integrated or included within a system, equipment, or device. It signifies that a particular function or component is available within the system itself. So, while both on board and onboard relate to inclusion or presence, they are used in different contexts and have distinct meanings.

Definition of on board:
phrase, on or in a ship, aircraft, train, or other vehicle
The captain announced that all passengers were on board, and the plane was ready for departure.
After a thorough discussion, the team decided to bring a new member on board to handle the marketing campaign.

Collocations with on board:
Some most commonly used collocations include:
1. Get on board: to join or support a particular cause, idea, or project.
We need everyone to get on board with the new company initiative for it to succeed.
2. Bring on board: to hire or include someone as a member of a team or organization.
The company decided to bring a seasoned expert on board to lead the research and development department.
3. Have on board: to have someone or something physically present within a vehicle or vessel.
We must ensure that we have all the necessary supplies and equipment on board before setting sail.
4. Keep on board: to retain or maintain someone or something within a particular group or organization.
The management decided to keep the talented employee on board despite the budget cuts.

Correct spelling, explanation: onboard describes integration or inclusion of features within a system or device, while on board deals with physical presence or inclusion within a group or vehicle. The choice between the two depends on the specific context and whether you are referring to built-in functionality or the physical state of being present. The combination of on and board in the word onboard signifies the idea of something being attached or integrated onto a flat surface or within a system. Over time, onboard has evolved to describe the inclusion or integration of something within a particular device, system, or vehicle.