Machi
Machi is the Japanese term for "waits". Given any mahjong hand at any instance, players are waiting for specific tiles either to develop their hands or complete it. Wait patterns arise during hand development, when mahjong tiles in possession in the hand are in need of other tiles in order to complete mentsu (melds) or the entire hand. Wait patterns during tenpai are of particular interest, simply because that is one of the main goals of hand development.
Mentsu
Hand development
Tenpai wait patterns overview
Many patterns arise during tenpai, such that they are named. These patterns may develop well before tenpai, but they are of particular interest during tenpai. After all, it is in the best interest of the player to know wait tiles during tenpai, in order to not miss it or even choose to change wait patterns, if needed.