1. The time it takes a planet to complete one orbit around its star. This is the most common definition. For example, Earth's planetary year is 365.25 days, which is the time it takes our planet to complete one orbit around the Sun. This is also sometimes called a sidereal year.
2. The time it takes a planet to return to the same position relative to the Sun and Earth. This is also called a tropical year. It is slightly shorter than a sidereal year because the Earth's axis is tilted and slowly precesses, causing the position of the Sun in the sky to drift slightly over time.
Therefore, the definition of "planetary year" depends on the specific context. When referring to a planet's orbital period around its star, it is usually the sidereal year that is meant. However, if you are talking about the time it takes for a planet to return to a specific position relative to Earth, then the tropical year is the relevant measure.
