Core Java refers to the fundamental aspects of the Java programming language and its standard libraries, which form the foundation for Java development. It includes essential concepts like object-oriented programming (OOP), basic data types, control structures, exception handling, and standard APIs for tasks such as I/O operations, networking, and multithreading. Core Java is platform-independent, enabling developers to write code that can run on any device with a Java Virtual Machine (JVM). Mastery of Core Java is crucial for developing robust and scalable Java applications.