Sea Organization

The Sea Organization is vaguely similar to a fraternal order in normal churches. All senior CofS orgs and corporations are staffed by SO members. 
An SO member signs a contract for a billion years, usually referred to as an SO contract. This has no legal standing, of course, and the weasel PR story is that it is just to symbolize the dedication which SO members have. Despite it having no legal meaning, most SO members are dead serious about honoring the terms of the contract when they sign it, and throughout their SO careers. In other words, an SO member intends to honor the contract for the remainder of this lifetime, then after a twenty-one year leave of absence rejoin for all of next lifetime, then take a twenty-one year leave of absence and so on.
Sea Org is so-called because Scientology once used boats that moved around on the sea as central headquarters, before LRH decided to move the headquarters to Clearwater.