Load factor of a hashtable (a) = # of keys / # of buckets
a directly regulates the time-complexity of a search (or insert) operation on a hashtable.
Though, the hashtable data structure gives a guarantee of O(1) insert/lookup time, an overloaded (a > 1) hashtable can result in O(n) operations (though the operations are also affected by the method of probing, viz. chaning or open addressing).
The expectation of a successful search for a key in a hashtable is given by :
E(successful probe) = 1 / (1-a)