a year is a unit of time, commonly understood as 365 days, which divides up the cycle of seasons into units. It is commonly used to measure age, to track time, or to denote anniversaries. It is also linked to the calendar, both solar and lunar.
1. Last year, I went on the most incredible vacation.
2. According to the old legend, they have been married for forty years.
3. She checked the date on the bottle—three years ago!
When used as a noun, a year is always plural, and is generally formed by adding -s to the end of the number, such as 'sixes' for six years. 'A year and a da(y)' or 'a year and two months' are also common usages.