Rho's best-known symbolic use has nothing to do with its ordinary sound, covered elsewhere in this atlas, and everything to do with its shape: overlaid on chi, the first two letters of the Greek Christos, Christ, rho's single descending stroke and chi's crossed strokes combine into the Chi-Rho monogram, one of the oldest and most widespread symbols of Christianity.
Early Christian writers Eusebius and Lactantius, writing in the 4th century, both record that the emperor Constantine adopted the sign after a vision before the Battle of the Milvian Bridge in 312 CE, and the monogram subsequently spread across Christian art, coinage, and manuscripts throughout the late Roman and Byzantine world.
In modern technical use, physicists and engineers reserve lowercase rho for density and for electrical resistivity, and mathematicians occasionally use it for correlation coefficients in statistics, none of which draws on either the Chi-Rho tradition or on rho's ancient Semitic root resh, meaning head. Astronomers, following the Bayer star-naming convention of 1603, also use Rho to mark a constellation's seventeenth- or eighteenth-brightest star, continuing the same ranked sequence used throughout the Greek alphabet.