NoClassDefFoundError is a runtime error in Java that occurs when the Java Virtual Machine (JVM) or a ClassLoader instance cannot find a class definition at runtime, even though it was available at compile time. This error often results from missing JAR files, permission issues, or incorrect classpaths, which can happen during normal method calls or when creating class instances. An example illustrating this error involves executing a Java program after renaming a class file, leading to the JVM failing to locate the class definition and throwing the error. To resolve this, ensuring the JAR file containing the missing class is in the classpath, checking for classpath overrides, verifying the Class-Path attribute in the manifest file, and recompiling the affected classes are recommended steps. Additionally, tools like Rollbar can help manage and analyze Java errors in real-time, offering a more confident approach to handling exceptions and deploying production code.