R.L. Trask 
Language and Linguistics: The Key Concepts [PDF ebook] 

A comprehensive and critical A-Z guide to the main terms and concepts used in the study of language and linguistics, definitions featured include: terms used in grammatical analysis branches of linguistics from semantics to neurolinguistics approaches used in studying language from critical discourse analysis to systemic linguistics linguistic phenomena from code-switching to conversational implicature language varieties from pidgin to standard language. This fully updated second edition includes a new introduction, a wide range of new entries (reflecting developments in linguistics) and added specialized further reading for lecturers and more advanced students.