seniority

Juniority is a antonym of seniority.

As nouns the difference between seniority and juniority

is that seniority is a measure of the amount of time a person has been a member of an organization, as compared to other members, and with an eye towards awarding privileges to those who have been members longer while juniority is the quality or state of being junior.

As nouns the difference between seniority and nonsenior

is that seniority is a measure of the amount of time a person has been a member of an organization, as compared to other members, and with an eye towards awarding privileges to those who have been members longer while nonsenior is someone who is not a senior.

As an adjective nonsenior is

not senior; lacking seniority.