Definition of Language（What is Language?) Based on Sapir， Hall Chomsky
It's a system of arbitrary vocal symbols used for human communication.
1 systematic 指語言的要素是按照規則組合在一起的。
2 arbitrary 語言符號是任意的，每一個語言象征和它所指的東西之間是沒有什么規則可循的，而是約定俗稱的。不同語言對同一種事物有不同的稱呼，該物體時不變的，而這種稱呼是任意的。“不管用什么稱呼，玫瑰的香味永遠是一樣的。”-莎士比亞
3 vocal symbols 發聲的語言系統，先有說，后有寫。
4 human-specific 人類所特有的，與動物的語言是totally different.
簡單地一句話概括了語言最基本的特征。
Design\defining features of human language
by Charles Hockett(American linguist)
1. Arbitrariness:
No logical connection between meanings and sonds.( 只有少數一些不是完全任意如onomatopoeic words(擬聲詞）photo 和 copy是arbitrary，但是photocopy就不是完全任意的。有少數compound words(復合詞)就不是完全任意的。)
意義：makes it possible to have an unlimited source of expressioms
2. Productivity/ Creativity
Peculiar to human languages, users of language can understand and produce sentences they have never heard before.(人可以說些他們從來沒有聽過也沒有見過的話。如 A red-eyed elephant is dancing on the hotel bed. 我們仍然是可以理解的。)
animals`s calls are limited.
3. Displacement:
Language can be used to refer to things which are present or not present, real, imagined matters in the past, present or future, or in far-away places.(把語言用來指不是眼前的東西，過去的東西，將來的東西,不存在的想象的東西，在很遠很遠地方的東西。)
4. Cultural tramission:
Language is culturally transmitted through teaching and learning, rather than by instinct.(語言文化的傳遞通過教與學。)
5.Duality: Lower level-sounds(meaningless);
High level-meanging(large units of meaning0
Functions of language
1. Phatic function--establsihing an atmosphere or maintaining social contact
2.Directive function--get the hearer to do something
3.Informative function--give information about facts
4.Interrogative function--get information from others
5.Expressive function--express feelings and attitudes of the speaker
6.Evocative function--creat certain feelings in the hearer(amuse, startle, sooth, worry or please)
7.Performative function--language is used to do things to perform actions