A language delay is a language disorder in which a child fails to develop language abilities at the usual age-appropriate period in their developmental timetable. It is most commonly seen in children ages two to seven years-old and can continue into adulthood. The reported prevalence of language delay ranges from 2.3 to 19 percent. Language is a uniquely human form of communication that entails the use of words in a standard and structured way. Language is distinct from communication. Communication is a two-stage process. The first stage is the process of encoding the message into a set of words (or signs in the case of Sign Languages) and sentence structures that convey the required meaning, i.e. into language. In the second stage, language is translated into motor commands that control the articulators (hands, face, body, lungs, vocal cords, mouth, tongue, teeth, etc.), thereby creating speech. Language delays are distinct from speech delays, in which the development of the mechanical and motor aspects of speech production are delayed. Many tend to confuse language delay with speech delay or even just late talker. All of these have different telltale signs and determining factors. Speech delay seems to be more similar to late talker compared to language delay. Speech is the verbal motor production of language, while language is a means of communication. Because language and speech are independent, they may be individually delayed. For example, a child may be delayed in speech (i.e., unable to produce intelligible speech sounds), but not delayed in language because they use a Sign Language. Additionally, language delay encompasses the entirety of language developmental progress being slowed and not just the speech aspects. Language delays are recognized by comparing language development of children to recognized developmental milestones. They are presented in a variety of ways, as every individual child has a unique set of language skills and deficiencies that are identifiable through many different screenings and tools. There are different causes leading to language delay; it is often a result of another developmental disorder and treatment requires analysis of the unique individual causes. The condition is frequently observed early on, among two- and three-year-olds. Early language delays are only considered risk-factors in leading to more severe language disorders.

Reference
Wiki: Delayed speech and language development