Chafing, according to Wikipedia. Not a complete definition, but a good "starter" definition.

The longer you run and/or the more you sweat, the more susceptible you are to chafing!

Some measures I take to prevent chafing are (in no particular order): athletic tape over the affected areas (particularly for the nipple area), Body Glide (or a similar anti-chafing stick), non-cotton fabrics (Dri-Fit or polyester materials do a LOT better than cotton).

Find out more at https://what-the-run.pinecast.co