Pay period
The window of time worked that a paycheck covers — biweekly = 14 days, semi-monthly = roughly 15 days.
The pay period is the window of time worked that a single paycheck covers. For biweekly payroll the pay period is exactly 14 days (two seven-day workweeks). For semi-monthly payroll the pay period is roughly 15 days, defined as either the 1st through the 15th or the 16th through the last day of the month. For weekly payroll it's seven days; for monthly, roughly 30 days.
The pay period is distinct from the workweek under the Fair Labor Standards Act. The FLSA workweek is a fixed seven-day period chosen by the employer for overtime computation; the pay period is the window covered by a single paycheck. Under biweekly payroll the two align cleanly (pay period = two workweeks). Under semi-monthly payroll they don't, and overtime has to be computed on the workweek and paid in the period containing the workweek's pay date.
Pay-period boundaries also drive time-card cutoffs, payroll-software pay-run dates, deduction calculations, and IRS deposit deadlines. Documenting your pay-period boundaries explicitly in the employee handbook avoids most of the common payroll disputes.