语言发育迟缓是指儿童语言发展的能力比同龄儿童水平低，或以较长时间在达到同样的语言能力。和语音发育迟缓不同，语言发育迟缓不包括发音的问题，而专指理解和运用语言的能力。即使儿童有正常的发音能力，亦可能因为脑部发展，难以辨认、理解和运用字词。语言和发音是两个不同的阶段，语言能力是指把听到或看到的句子辨认和理解，以及把脑里所想的变成字句，而发音是透过器官去制造字词的声音。

一些儿童可能只有语音发展迟缓，但没有语言发展迟缓的问题。这类儿童会尝试说话，但所说的话其他人会听不懂。但有语言发展迟缓问题的儿童，却很可能因为无法把语音和事物链接起来，而同时有语音发展迟缓的问题。

Language delay is a failure in children to develop language abilities on the usual age appropriate for their developmental timetable. Language delay is distinct from speech delay, in which the speech mechanism itself is the focus of delay.

Oral communication is a two-stage process. The first stage is to encode the message into a set of words 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 (organs and structures such as the lungs, vocal cords, mouth, tongue, teeth, etc.), thereby creating speech, i.e. orally-expressed language.