In the late Roman republic, the cohort (of which there were between six and ten) became the basic tactical unit of the legions. The cohort was composed of five to eight centuries, each led by a centurion assisted by an optio, a soldier who could read and write. The senior centurion of the legion and commander of the first cohort was called the primus pilus, a career soldier and advisor to the legate. While every normal cohort was composed of five to eight centuries (normally six in secondus to decius cohorts), the one that was led by the primus pilus (the first) had about ten centuries, or 800 men. It also had a number of other staff, such as cooks, clerks, etc; that is, non-combatants.
Only eight officers in a fully officered legion outranked the primus pilus: The legate (lēgātus legiōnis), commanding the legion; the senior tribune (tribunus laticlavus); the Camp Prefect (praefectus castrorum); and the five junior tribunes (tribūnī angusticlāviī).
Contrarily to a common misconception, the literal translation of "primus pilus" is not "first spear", after the roman word pilum. If it was so, the rank would be called "primus pilum" (nominative case, 2nd declination, neuter), while the correct term "primus pilus" (nominative case, 2nd declination, masculine) refers to the alternate traditional name for a Triariimaniple. As such the "Centurio Primus Pilus" litteraly translate as the Centurion of the First Triarii Maniple.
^A.Liberati and E.Silverio - Vita e Costumi dei Romani Antichi - Vol.5 Organizzazione Militare: Esercito - Edizioni Quasar - Roma 1988 - for the Museo della Civiltà Romana, chapter 2 page 21. See also  and