A junior is a person in the third year at an educational institution in the US and some other countries, usually at a secondary school or at the college and university level, but also in other forms of post-secondary educational institutions.

But generally speaking, juniors in high school are typically 16-17 years old. Most students enter high school at age 14 or 15 and follow a four-year curriculum before graduating. Juniors are considered to be in the third year of high school, with seniors being in their final year.

: a student in the next-to-last year in a high school, college, or university

A junior in high school is typically in 11th grade. It's the third year out of the four-year high school experience, with freshmen being in 9th grade, sophomores in 10th, juniors in 11th, and seniors in 12th.

11th Grade: Ages 16-17 (Third Year of High School). 11th grade students are referred to as “juniors.” 12th Grade: Ages 17-18 (Fourth and Final Year of High School). 12th grade students are referred to as “seniors” and often fall prey to “senioritis”—particularly after college admission decisions are released.