The Natural Language Toolkit, often abbreviated as NLTK, is a popular library in the Python programming language designed for working with human language data, or natural language processing (NLP). It provides easy-to-use interfaces and tools for tasks such as tokenization, parsing, classification, and stemming, making it accessible for both beginners and experienced developers. NLTK includes a variety of corpora and lexical resources, such as WordNet, which help users analyze and manipulate text. It is widely used in academia and industry for research, teaching, and developing applications that require understanding and processing of human language.