A no-show is a client who does not come to an appointment and does not tell you. The time is lost, any product prepared for them is wasted, and the person who could have had the slot was turned away. A no-show is different from a late cancellation, because the late canceller at least gave you a chance to fill the time.
Most no-shows come from forgetting rather than rudeness, which is why reminders work. Many of the rest are clients who booked casually, often a long way ahead, with no money down.
In practice
A massage clinic does 140 appointments a month at an average of $105. In one month it records 7 no-shows: 5% of bookings and $735 of lost time. It then starts sending a reminder text the day before every appointment and asks new clients for a $30 deposit. The next month has 2 no-shows, or $210 of lost time. Both were new clients, so their deposits were kept and the actual loss was $150.
Decide what you do after a no-show before it happens. A first one from a long-standing regular might be waived with a friendly message. A second, or one from a new client who paid a deposit, follows the rule. Keep a simple count per client in the diary notes. If no-shows climb again, look first at how far ahead people are booking and whether every booking received a confirmation.
There is a fuller plan in no-shows and deposits.