A junior is a student in their third year of study (generally referring to high school or college/university study) as coming immediately before their senior year. Juniors are considered upperclassmen.
In the United States, the 11th grade is usually the third year of a student's high school period and is referred to as junior year. High school juniors are advised to prepare for college entrance exams (ACT or SAT) and to start narrowing down on colleges they want to go to.
In the U.S., colleges generally require students to declare an academic major by the beginning of their junior year. College juniors are advised to begin the internship process and preparing for additional education (medical school, law school, etc.) by completing applications and taking additional examinations.