In mathematics, the slope or gradient of a line is a number that describes both the "direction" and the "steepness" of the line. Slope is often denoted by the letter "m"; there is no clear answer to the question why the letter "m" is used for slope, but its earliest use in English appears in O'Brien (1844) who wrote the equation of a straight line as and it can also be found in Todhunter (1888) who wrote it as ""y" = "mx" + "c"".