Definition
How often a muscle or a lift is trained each week.
Twice a week per muscle beats once at the same volume for most people, and three times helps the skill side of barbell lifts. Frequency is mostly a way to spread volume: 16 sets of chest as two sessions of 8 keeps the quality higher than one session of 16.
Splits follow the days available. Two days: full body twice. Three: full body, or upper, lower, full. Four: upper/lower. Five or six: push, pull, legs.
Programme the frequency the client can show up for, not the ideal one. Three sessions done beats five planned.