Generally, the population of keys for a hashed list is greater than the storage area for the data. For example, if we have an array of 50 students for a class in which the students are identified by the last four digits of their National Identity Card Numbers, then there are 200 possible keys for each element in the array (10,000/50).
Because there are many keys for each index location in the array, more than one student may hash to the same location in the array. We call the set of keys that hash to the same location in our list synonyms.
Hash function H (K) are used to map keys into table addresses to store data as table entries (K, I) in hashed table. Almost always we use hashing techniques in which there are many more distinct keys K than there are table addresses, so we encounter a situation in which two distinct keys, K1 ¹ K2, map to the same table address, i.e.;
H (K1) = H (K2)