a : a punctuation mark : used chiefly to direct attention to matter (such as a list, explanation, quotation, or amplification) that follows
b : the sign : used between the parts of a numerical expression of time in hours and minutes (as in 1:15) or in hours, minutes, and seconds (as in 8:25:30), in a bibliographical reference (as in Nation 130:20), in a ratio where it is usually read as "to" (as in 4:1 read "four to one"), or in a proportion where it is usually read as "is to" or when doubled as "as" (as in 2:1::8:4 read "two is to one as eight is to four")