Definitions of basis
the important facts and ideas from which something is supported and developed
The story contains a thinly disguised attack on my client which has absolutely no basis in fact.
the point from which calculations are made
Seats were allocated, on a top-down basis, in proportion to the parties' share of the vote.
the reason why a person takes a particular action
The Fees Office refused to pay the full £1,699 he asked for on the basis that the maximum claim was £750.