# Glossary

**dictionary:** A mapping from a set of keys to their corresponding values. 

**key-value pair:** The representation of the mapping from a key to a value. 

**item:** Another name for a key-value pair. 

**key:** An object that appears in a dictionary as the first part of a key-value pair. 

**value:** An object that appears in a dictionary as the second part of a key-value pair. This is more specific than our previous use of the word "value". 

**implementation:** A way of performing a computation. 

**hashtable:** The algorithm used to implement Python dictionaries. 

**hash function:** A function used by a hashtable to compute the location for a key.

**hashable:** A type that has a hash function. Immutable types like integers, floats and strings are hashable; mutable types like lists and dictionaries are not. 

**lookup:** A dictionary operation that takes a key and finds the corresponding value. 

**reverse lookup:** A dictionary operation that takes a value and finds one or more keys that map to it. 

**singleton:** A set (or other sequence) with a single element. 

**histogram:** A histogram is an accurate representation of the distribution of numerical data. In simpler terms, it could be seen as a set of counters in a program.

**global variable:** A variable defined outside a function. Global variables can be accessed from any function. 

**flag:** A boolean variable used to indicate whether a condition is true. 

**declaration:** A statement like `global` that tells the interpreter something about a variable.