A hybrid could be any combination of two things that are normally considered separate.
In the animal kingdom you could call naturally-occuring cross-bred creatures the hybrid offspring (such as two different bird species, or a lion and a tiger having cubs).
In the motoring world, the most common usage of the word hybrid is to simply denote the combination of a combustion power unit with an electric power unit to drive the vehicle. We use the acronym HEV to identify a basic hybrid electric vehicle.
As inventions go, the idea of the HEV is so old it predates the moving assembly line.
In about 1899 Ferdinand Porsche developed the System Lohner-Porsche Mixte (following work on an all-electric vehicle that used a battery and hub-mounted electric engines, demonstrating that most electric drivetrain concepts are also quite old). The Mixte was on the market from 1900 to 1905, and other examples from that period were sold to the public or raced in motorsport as well.
Hybrid is actually a pretty broad term for vehicles though, because not all hybrids are the same. Far from it.
Some hybrids very much prioritise the electrical source of motive force, with the combustion engine there merely as assistance and to recharge the battery on the go. In such cases, internal combustion is only used as a means for keeping the battery charged or to supply energy directly to the electric engine.
The Mixte was like this, with a petrol engine driving a generator for a modest battery or directly powering the hub-mounted electric engines in the front wheels.
In some hybrids it will be easy to recharge the battery from the mains just like a battery-only electric vehicle (BEV). These plug-in hybrid electric vehicles get the acronym PHEV.
Other hybrids very much prioritise the combustion engine as the means for propulsion and the electric unit is either merely assisting sometimes, or the electric capability might only last for a short (sometimes extremely short) distance before the fossil fuel burner needs to be fired up.
Formula 1's rules for vehicles from 2009 were like this. These rules allowed for a kinetic energy recovery system (KERS for short) of some sort (although it could be a flywheel, or a battery, or something else, so they weren't called hybrids), and that could be deployed by the driver for a total of no more than 6.67 seconds per lap to increase the total power output by 60kW. This was used either to accelerate harder out of some corners, or occasionally to help with an overtake.
Formula 1's hybrid-era engines introduced in the 2014 season still very much prioritise the combustion unit, but their electric propulsion is configured in a way that allows it to move the car (as demonstrated occasionaly by a driver bump-starting their car after a stalled engine) and can be deployed for as long as the battery has some charge to offer. They also have settings to choose from where it can be deployed automatically or by the driver.
Meanwhile some (or all) of the hybrids in the World Endurance Championship over a similar time period have connected the combustion powertrain to the rear wheels, and the electric motivation to the front wheels. As seen at Le Mans though, the LMP1-era cars didn't have enough battery capacity available to complete a full lap if the combustion engine failed or ran out of fuel.
As for road cars, so far as I can tell the best sort of hybrid to consider would be a PHEV with a large battery capacity and a clear prioritisation of electricity for motive power. These are the kind that only really use the petrol engine as a backup. Most of the time you can treat it like a BEV, but if you are unable to recharge the battery from an external source for some reason the internal combustion is activated and you keep going.
However, such PHEVs are still a compromise. The equivalent space taken by the combustion system (including its fuel tank) could instead accommodate more battery capacity to increase the vehicle's range that way. And I suspect this is part of the reason why BEV sales nearly doubled in Australia in 2021 compared to previous years, whereas sales of hybrids stayed roughly the same.
