What is Seniority?

Legal Definition
In finance, seniority refers to the order of repayment in the event of a sale or bankruptcy of the issuer. Seniority can refer to either debt or preferred stock. Senior debt must be repaid before subordinated (or junior) debt is repaid. Each security, either debt or equity, that a company issues has a specific seniority or ranking. Bonds that have the same seniority in a company's capital structure are described as being pari passu. Preferred stock is senior to common stock in a sale when preferred shareholders must receive back their preference, typically their original investment amount, before the common shareholders receive anything.
-- Wikipedia
Legal Definition
Attained by the employee who has worked the longest with the firm. Often orders promotion and lay offs and benefits to employees.