Before going to start with official definition just wanted to tell you about Groovy in my personal view.
Groovy can fit in to any kind of APIs in java to make more simple and dynamic. For example with maven If you want to make your own logic inside, can be done by Groovy.
Like ways can use Groovy with SoapUI, MuleESB and basic code of java.
Groovy is an object-oriented programming language for the Java platform. It is a dynamic language with features similar to those of Python, Ruby, Perl, and Smalltalk. It can be used as a scripting language for the Java Platform, is dynamically compiled to Java Virtual Machine (JVM) bytecode, and interoperates with other Java code and libraries. Groovy uses a Java-like curly-bracket syntax. Most Java code is also syntactically valid Groovy
Groovy is a powerful, optionally typed and dynamic language, with static-typing and static compilation capabilities, for the Java platform aimed at multiplying developers' productivity thanks to a concise, familiar and easy to learn syntax. It integrates smoothly with any Java program, and immediately delivers to your application powerful features, including scripting capabilities, Domain-Specific Language authoring, runtime and compile-time meta-programming and functional programming.
Most valid Java files are also valid Groovy files. Although the two languages are similar, Groovy code can be more compact, because it does not require all the elements that Java requires. This makes it possible for Java programmers to gradually learn Groovy by starting with familiar Java syntax before acquiring more Groovy idioms.
Groovy provides native support for various markup languages such as XML and HTML, accomplished via an inline DOM syntax. This feature enables the definition and manipulation of many types of heterogeneous data assets with a uniform and concise syntax and programming methodology.