Literally translated "People Square", the name Piazza del Popolo might have a different origin.The word popolo can be derived from latin populus (italian pioppo, english poplar), since traditionally there used to be a small wood of poplars around the tumb of the emperor Nero, which was nearby.
Anyway, it is confirmed that pope Pasquale II (11th century a.D.) issued the construction of a chapel (located where now the S. Maria del Popolo church is located, behind the obelisk in the photo) relying on people's expenses.