Year
A year is the time a planet takes to complete one orbit around the Sun; Earth takes about 365.25 days, which is why leap years exist.
Definition
A year is how long it takes Earth to travel all the way around the Sun, about $365$ days and $6$ hours, and every four years we add an extra day, February 29, to use up those leftover hours. More generally, a year is the period of a planet's revolution around the Sun; Earth's year of about $365.25$ days carries it through the full cycle of seasons and the complete sequence of seasonal constellations, and other planets have years set by their distance from the Sun following Kepler's third law. Astronomers define several years. The sidereal year, one revolution relative to the fixed stars, is $365.2564$ days; the tropical year, from one March equinox to the next, is $365.2422$ days and is shorter because precession moves the equinox westward about $20$ minutes per year, and this is the year that calendars track so that seasons stay aligned. The anomalistic year, perihelion to perihelion, is $365.2596$ days, and the Gregorian calendar's average year of $365.2425$ days approximates the tropical year to within $27$ seconds.
Example
If you are $12$ years old, you have ridden around the Sun $12$ times, and each trip is about $940$ million kilometers long. Mars is $1.52$ AU from the Sun and its year is $687$ Earth days; Jupiter at $5.2$ AU takes $11.9$ Earth years, and Saturn at $9.5$ AU takes $29.5$ Earth years. By Kepler's third law, a planet at $a = 9.54$ AU has $T = 9.54^{1.5} \approx 29.5$ years, Saturn's period. An exoplanet orbiting a Sun-like star at $0.05$ AU would have a year of only $0.05^{1.5} \approx 0.011$ years, about $4$ days.
Key Insight
A year is different on every planet: a year on Mercury lasts only $88$ Earth days, but a year on Neptune lasts $165$ Earth years. The leap year rule has an extra twist, since years divisible by $100$ are not leap years unless divisible by $400$, so 1900 was not a leap year but 2000 was; the rule matches the calendar to the true length of the year to within one day in about $3{,}000$ years. The Julian calendar's $365.25$-day year drifted $11$ minutes per year from the tropical year, which accumulated to $10$ days by 1582. The Gregorian reform dropped those days and adjusted the leap rule, a reminder that calendars are astronomy made practical.