a base refers to the numeric power which a base number is raised to. eg the number 61 to the base 8 would be (in base 10 or decimal) 6 times 8 to the power 1 (=48) plus 1 times 8 to the power zero (=1). Thus 48+1=49. Hexadecimal is to the base 16 and it gets round the fact that decimal users would have difficulty with the digits by introducing alphanumeric characters eg 0-9 and A-F ..
sorry for pedantry ... the nautical mile was standardised around the time the UK decimalised ....