Aside from their literal meanings, junior and senior have an array of connotations related to hierarchy.
Junior, from the Latin term juvenis, from which juvenile is also derived, refers to someone younger than another. It also applies to a young person or, more specifically, a son. Until well into the twentieth century, a boy or a young man might be addressed as Junior (though it was generally considered derogatory or at least condescending when directed at an adult), and the tradition persists of appending the abbreviation Jr. (no intervening comma is necessary) to the name of a male child who shares his father’s exact name.
Junior also applies to academic standing; in a four-year collegiate or secondary school system, a junior is someone in the third of four years of study. Schools for students in grades seven through nine (formerly grades seven and eight) in a K–12 system are often labeled “junior high schools.”
The word can also refer to someone of inferior rank (“lieutenant junior grade,” for example, as opposed to a full lieutenant, or “junior account executive”) or, in fashion, a clothing category for slender women and girls.
As an adjective, junior means “younger,” “youthful,” “more recent” (with a connotation of inferiority or subordination), “lower in rank,” or “on a smaller scale.” It also applies to class standing or, as part of the phrase “junior varsity,” an athletic team subordinate to the varsity, or the primary team.
Senior, borrowed directly from Latin and meaning “older,” is related to senile and senescence but has usually more positive connotations than those cognates. It refers to someone older than another or of higher rank. A senior in college or high school is in the final year of study, and senior might also refer, in an academic context, to a high-ranking fellow at a university.
The abbreviation Sr., following a name (again, with no intervening comma), indicates that the man so named has a son with the exact same name.
As an adjective, senior designates someone or something as having been born, or established or enrolled, before another, or being of higher rank. (Some military hierarchies have, for example, senior captains, who rank above captains.)
Senior has also become a synonym for elderly with what is widely considered a more positive connotation; it’s a truncation of “senior citizen.” It’s applied in phrases such as “senior center” and “senior rights.”
Like junior, senior can have a derogatory connotation, though, as in “senior moment,” a light-hearted reference to forgetfulness as a symptom of aging, it is usually not meant to insult. But take care with using either term to note, respectively, someone’s youth or inexperience or their age.