Prōtos (πρῶτος) is the superlative form of pro (before), meaning first — in terms of time (the first in a series), rank (the chief, most important), or priority. It appears about 98 times in the NT. Its antonym is eschatos (G2078, last).
The word is used for the first day of the week (the day of resurrection), the first commandment, the first born, and crucially for Jesus himself as the "first and the last" — a divine title borrowed from Isaiah's portrait of YHWH.
The prōtos/eschatos axis is one of the NT's most profound theological structures. Jesus subverts human hierarchies: "Many who are first [prōtos] will be last [eschatos], and the last first" (Matt. 19:30). The Kingdom of God reverses worldly ranking — greatness is measured by servanthood, glory by sacrifice. The rich young ruler, first in worldly terms, leaves last; tax collectors and prostitutes enter the Kingdom before the religious leaders.
Yet Christ himself is the eternal prōtos. He is "the firstborn [prōtotokos] over all creation" (Col. 1:15), "the firstborn from the dead" (Col. 1:18), the Alpha and Omega (Rev. 1:8), the First and the Last (Rev. 1:17). His resurrection establishes him as the first of the new humanity — and because he is first, we follow. The greatest commandment (prōtē entolē, Mark 12:28-30) is to love God with everything, which is both the first priority and the foundation of all others.