sen·ior 
(sēnyər)
(sēnyər)adj.
1. Abbr. Sr. Of or being the older of two, especially the older of two persons having the same name, as father and son.
2. Of or relating to senior citizens.
3.
a. Being in a position, rank, or grade above others of the same set or class: a senior officer; the senior ship in the battle group.
b. Having precedence in making certain decisions.
4. Of or relating to the fourth and last year of high school or college: our senior class.
5. Relating to or being a class of corporate debt that has priority with respect to interest and principal over other classes of debt and equity by the same issuer.
n.
1.
a. A person who is older than another: She is eight years my senior.
b. A senior citizen.
2.
a. One that is of a higher position, rank, or grade than another in the same set or class.
b. A student in the fourth year of high school or college.
[Middle English, from Latin, comparative of senex, old; see sen- in the Appendix of Indo-European roots.]
Usage Note: The Oxford English Dictionary traces the use of senior in the sense of "an older person" to the 15th century. In contemporary American English, however, this sense of senior is generally taken to be a shortening of the more recent senior citizen, and those who object to the compound may object to the shorter term as well. However, as the OED makes clear, senior has always evoked the positive qualities of aging, including wisdom, dignity, and superior position, and it is less likely to sound condescending than the obviously coined term senior citizen. In any case, there is often no clear alternative to senior other than constructions such as older person or older adult. See Usage Notes at elderly, old.