A lump sum payment is simply a lump sum: a single amount paid in full at one time, rather than through a series of smaller instalments. The two terms are used interchangeably in banking, lending and insurance; "lump sum payment" emphasises the act of paying, while "lump sum" more often describes the amount itself.
The phrase is common in loan documentation, where a borrower may be offered the choice between regular monthly instalments and a single lump sum payment to clear a balance early, and in pension and insurance paperwork, where a policyholder may elect a lump sum payment instead of ongoing periodic payments.
For how lump sums are used, how they compare with instalment payments, and the tax and planning issues they raise, see the main Lump Sum entry.