Is there a legal term for a waiver by inaction?

so for example if an employee under contract is prohibited from doing certain things for example writing in the local news paper (or something) but does so anyway and the employer finds out and does not fire him.The employee continues to write in the local paper for years and the employer doesn't fire him even though he knows what the employee is doing. One day the employer decides to fire the employee and cites his writing in the paper as the reason for the firing. Can the employee argue that the employer has waived his right to fire him on those grounds because of the employer's inaction from the first offense? and is there a legal term from a waiver by inaction?