In object-oriented programming

Object-oriented programming

Object-oriented programming is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction,...

 with class

Class (computer science)

In object-oriented programming, a class is a construct that is used as a blueprint to create instances of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members which enable these class instances to have state and behavior...

es, an instance variable is a variable

Variable (programming)

In computer programming, a variable is a symbolic name given to some known or unknown quantity or information, for the purpose of allowing the name to be used independently of the information it represents...

 defined in a class (i.e. a member variable

Member variable

In object-oriented programming, a member variable is a variable that is associated with a specific class, and accessible for all its methods. If there is only one copy of the variable shared with all instances of the class, it is called a class variable or static member variable...

), for which each object

Object (computer science)

In computer science, an object is any entity that can be manipulated by the commands of a programming language, such as a value, variable, function, or data structure...

 of the class has a separate copy. They live in memory for the life of the object.

An instance variable contrasts with a class variable

Class variable

In object-oriented programming with classes, a class variable is a variable defined in a class of which a single copy exists, regardless of how many instances of the class exist.A class variable is the opposite of an instance variable...

, and it is a special type of instance member. An example (C++ or Java) declaration of an instance variable is private double length.

Technically speaking, instance variables are objects stored in individual states in "non-static fields", that is, fields declared without the static keyword. Non-static fields are also known as instance variables because their values are unique to each instance of a class (to each object, in other words); the currentSpeed of one bicycle is independent from the currentSpeed of another.
A simpler definition is that instance variables are things an object knows about itself.

A typical declaration in Objective-C would look like:

@interface CustomClassName : NSObject{ NSString *ivar; }