A hash table is a very fast kind of lookup table, somewhat like an alist in that it maps keys to corresponding values, but much faster. The printed representation of a hash table specifies its properties and contents, like this:
⇒ #s(hash-table size 65 test eql rehash-size 1.5
rehash-threshold 0.8125 data ())
See Hash Tables, for more information about hash tables.