what periodic motion is used as a reference for our year
The Earth's orbit of the Sun. I'm sure you knew that.
As usual, things are more complicated. What's a year?
On way of defining it is the time that it takes for the sun the complete exactly the winter-spring-summer autum cycle, to be back exactly in the same position. That's a solar year.
Another way of defining it is when the Earth has completed a full orbit around the sun, and all the stars are back in the same position. That's a star year, or according to the specialists, a sidereal year.
Now, a sidereal year is about 20 minutes longer than a solar year. The difference is caused by the precession of the equinoxes, where the spin axis of the Earth moves slighly in a cone or circle, to be completed in some 26,000 years.
If we would use the sidereal year as a standard year, the seasons would slowly shift due to that precession, so our standard year is a solar year.
A sidereal year is approximately 365.256363 days (between 1994-2000), as said, slightly longer than the solar year of 365.2425 days.
We also have
The Anomalistic Year, the time from one perihelion passage (smallest distance to the sun) to another for the Earth.
A eclipse year, the time between node passages of the moon.
A tropical year, The mean interval between vernal equinoxes, or the moments the Spring starts.
Separate names with a comma.