The first definition of colon in the dictionary is -lons. the punctuation mark :, usually preceding an explanation or an example of what has gone before, a list, or an extended quotation. Other definition of colon is -lons. this mark used for certain other purposes, such as expressions of time, as in 2:45 p.m., or when a ratio is given in figures, as in 5:3. Colon is also -la. a part of a rhythmic period with two to six feet and one principal accent or ictus.