se ‧ ni ‧ or ‧ i ‧ ty /ˌsiːniˈɒrəti, ˌsiːniˈɒrɪti $ -ˈɔː-, -ˈɑː-/ BrE AmE noun [uncountable]
1 . if you have seniority in a company or organization, you have worked there a long time and have some official advantages:
I had 15 years seniority, and they couldn’t fire me.
2 . when you are older or higher in rank than someone else:
a position of seniority