"Power Chord" usually means the root and the fifth interval played together and sometimes the octave of one or both added. Eg, the lowest two notes of the "E" shaped barre chord, or the lowest two notes of the A shaped barre chord are the most common. It's a standby of a zillion rock bands and lots of classic rock songs. Most often, but not always, the root is the lowest pitch of the chord. It's an easy movable chord without the third so it's neither major or minor. I prefer the term "5 chord", but to each his own.