Natural language processing (NLP) is a field of computer science

Computer science

Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...

 and linguistics

Linguistics

Linguistics is the scientific study of human language. Linguistics can be broadly broken into three categories or subfields of study: language form, language meaning, and language in context....

 concerned with the interactions between computers and human (natural) languages; it began as a branch of artificial intelligence.
In theory, natural language processing is a very attractive method of human–computer interaction

Human–computer interaction

Human–computer Interaction is the study, planning, and design of the interaction between people and computers. It is often regarded as the intersection of computer science, behavioral sciences, design and several other fields of study...

.
Natural language understanding

Natural language understanding

Natural language understanding is a subtopic of natural language processing in artificial intelligence that deals with machine reading comprehension....

 is sometimes referred to as an AI-complete

AI-complete

In the field of artificial intelligence, the most difficult problems are informally known as AI-complete or AI-hard, implying that the difficulty of these computational problems is equivalent to solving the central artificial intelligence problem—making computers as intelligent as people, or strong...

 problem because it seems to require extensive knowledge about the outside world and the ability to manipulate it.

Whether NLP is distinct from, or identical to, the field of computational linguistics

Computational linguistics

Computational linguistics is an interdisciplinary field dealing with the statistical or rule-based modeling of natural language from a computational perspective....

 is a matter of perspective.