Each month, your hair grows at a certain rate that may increase or decrease slightly from month to month. This rate is typically 1/2 inch per month: slightly faster for Asians and Caucasians and closer to average or just below for those of African descent. This regular monthly growing happens for a number of years for each strand of hair on your head. Each strand has its own sort of timetable set around this. Both your monthly growing rate and your total growing period, or phase, are determined to a large extent by your genetics. Environmental and personal health factors may also influence both rates.
Your terminal length is the longest length that any hair on your head can grow given your monthly growing rate and the length of your growing periods. Simply put, terminal length is the length that your hair would reach if it were never cut, never broke, and was just allowed to grow freely without interruption. Once a hair has completed its total growing period and has reached its terminal length, it will shed naturally and be pushed out by (or eventually be replaced by) a new, growing strand. This process repeats all over your head, day in and day out, for your entire lifetime for each single strand of hair.
Hair that has reached its terminal length tapers naturally into this position. Hemlines that have reached terminal length are rarely blunt or even because different strands of hair are at different points in their life spans and rarely reach terminal length together. *(If all of your strands of hair reached terminal length together, your hair would fall out all at once, you’d be bald for a few weeks, and then your hair would start growing in all at the same exact time. Not a good look.)