A pension plan that has sufficient assets needed to provide for all accrued benefits. In order to be fully funded, the plan must be able to make all the anticipated payments to pensioners. A plan’s administrator is able to predict the amount of funds that will be needed on a yearly basis; a determination can be made regarding the financial health of the pension plan.
Explained further: A fully funded pension plan is one that has the financial stability to make current and future benefits payments to pensioners. The plan depends on capital contributions and returns on its investments to achieve stability. Companies distribute annual benefits statements specifying whether or not the pension plan is fully funded. As such, employees can determine the financial strength of the plan.
Definition courtesy of Investopedia