For code purposes, see 402(e)(4)(D)(i)
(D) Lump-sum distribution
For purposes of this paragraph—
(i) In general The term “lump-sum distribution” means the distribution or payment within one taxable year of the recipient of the balance to the credit of an employee which becomes payable to the recipient—
However, you should refer to any pertinent language in the plan document that my address a "lump-sum" in the scenario you describe.