The principle of a "Princeps"(= First citizen) was well known before the Principate, althoughto my knowledge only in an unofficial connotation. It was simply applied to the most dominant (for whatever reason) man in Roman politics. Supposedly the eagerness to reach this status was what partially fueled many rivalries (Marius vs Sulla, Caesar vs Pompey) or fueled many factional conflicts (trying to prevent a single man from obtaining or keeping this status, like ie Scipio).
Only under the Principate the term Prrinceps became part of the official titulature.
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