Essentially, a year is the time it takes for the Earth to go around the Sun. So yes, Venus has a Venusian year (0.6a) and Mars has a Martian year (1.88a). But for the purpose of today, I'm going to talk about how the Year is a standard unit of measurement.
Astronomers need a way of measuring things. Since there's no standard galactic measuring stick for, well, everything, we've taken what's most familiar and made that our basis. For example, the mass of planets is measured by the mass of the Earth (M⊕) and the mass of stars is measured by the mass of the Sun (M☉). Short distances are measured by Astronomical Units (AU), which is the mean distance from the Earth to the Sun and long distances are measured by lightyears (ly) (the distance it takes for light to travel a year).
In astronomy, one measurement of time is the Julian year (symbol: a), which is exactly 86,400 seconds (as seconds are the base unit of time in SI). This equates to about 365.25 days, if that makes your brain hurt less. That's a very familiar number, with our calendar years being 365 days, except for every four years, when we add up the .25 of a day, and tack on an extra Leap day, so our days can sync up with our years. Our current Gregorian Calendar is based off this cycle.
While we've known about this extra quarter-day for a few thousand years, we didn't realise exactly how precise we'd not calculated it, so our earlier calendars had a bit of drift going on, and occasionally needed serious correction. That's why the ten days of Oct 5-15 1582 AD (CE) don't actually exist. Also why Ramadan appears to drift in relation to our civil calendars. And if you were born in Sweden in February 30, 1712, I am very, very sorry for you. Here's three hundred years' worth of birthday cake to make up for that double-leap day.
Let's put calendars aside and talk astronomy.
Julian years are used to measure duration. For example, how long would it take light to reach us from Alpha Centauri? About 4.6 years. (A Julian year is what they use to calculate a lightyear.)
How long does it take for Jupiter to go around the Sun (aka a Jovian year)? 11.8618 (Julian) years.
There's other types of years such as the sidereal, tropical and draconic years, used to measure stuff in relation to Earth but for general astronomical purposes in measuring duration in the rest of the Universe, we prefer the Julian year. Feel free to go hardcore if you wish regarding the other year types.
Her Grace does not mind collecting years as she goes along. Old age is a privilege denied to many.