Wage growth is your estimate of the rate that salaries will increase over time.
The long term rate of General Schedule salary increases used by the Civil Service Retirement Fund Board of Actuaries is 3.75%. The Ballpark Estimate uses 3.75% as a default rate, but will allow you to choose any rate between 0% and 10%.
You should choose assumptions--or range of assumptions--that are right for you based on your particular circumstances.