For terms that aren't listed here, refer to the Oracle Java Glossary of Terms.

class: A blueprint, or template, for an object. Classes define properties (data) and methods (behavior) that each object created from the class template will have. In Java, a publicly available class is declared as follows:

public class MyClass {
    // property and method definitions

classpath: A parameter for the Java Virtual Machine or Java compiler that specifies the location of classes and packages available for use.

Source: Wikipedia

declaration: To specify the name and data type of a variable or property. This may be done either in conjunction with or prior to initialization.

// Declaration without initialization
String name;
// Declaration with initialization
String name = "Michael";

encapsulation: The bundling of related data and behaviors that operate on that data, usually with restricted access to internal, non-public data and behaviors. In object-oriented programming, encapsulation is achieved through the use of objects.

Source: Wikipedia

Integrated Development Environment (IDE): An application that provides access to and integration between software development tools that would otherwise be used independently, such as compilation, execution, access to documentation, built tools, source code management, and version control.

inheritance: A mechanism within object-oriented programming that allows one class to be based on another class, this "inheriting" its properties and behaviors. Inheritance is also sometimes referred to as subtyping or extension.

Source: Wikipedia

initialize: To assign a value to a declared variable or property for the first time. This may be done in conjunction with or after declaration.

instance: An explicit object within a program.

instance property (or instance variable): A property of a class that is non-static, so that each instance of the class has its own version of the property.

JAR file: JAR stands for "Java Archive" and is a file format that packages Java classes along with other relevant files (images, template files, etc) into a single file for distribution. In IntelliJ, you will see any JARs that are available to your project under External Libraries in the Project pane.

object: A value (i.e. a concrete unit within a program) created from a class. An object has a data type, which is the same as the name of the class it was created from.

MyClass anObject = new Myclass( /* constructor parameters */ );

package: A collection of class organized within a namespace. Packages also allow for controlled access of class members within the class. Packages are also used to organize code with related functionality.

Source: Wikipedia

polymorphism: An object-oriented mechanism that allows for objects of different types to be used in the same way.

primitive: A built-in non-class type. Common primitives include int, double, char, and boolean. Each primitive type has a corresponding class type (e.g. Integer, Double, and so on).

Source: Oracle