Various unsubstantiated theories exist regarding the use of the letter "m" to denote the slope in the straight-line equation, but the actual reason is unknown. Some believe that the mathematician Descartes used "m" for slope for the French word "monter," which means to climb. However, there is no evidence that he utilized this letter symbol for slope.
Another suggestion made by John Conway was that this letter symbol for slope may refer to "modulus of slope." Some mathematics historians believe that the use of this letter symbol in the straight-line equation occurred over time. For example, the first appearance in print of the letter "m" for slope was in 1844. Through subsequent years, different textbooks also used other letter symbols, such as Swedish textbooks representing slope by the letter "s."
The straight line equation, also called the slope-intercept form, is given as y = mx + b, where b is the y-intercept.