Cyclometric complexity is a concept in Complexity theory. Its magnitude is denoted by the letter ("M"). "M" is defined to be one larger than the number of decision points (IFs, UNTILs, ENDs...) in a module (function, procedure, chart node, etc.), or more generally a system.
McCabe's rule of thumb is that "modules" with M of ten or more are error-prone. CASE structures are an exception. Due to their structure, they can have more than ten decisions without increasing the likelihood of errors.