Descriptivism - Observation of how language is used. This is generally the method used in linguistics. (Cf. prescriptivism)
Morpheme - An individual unit of meaning in a word. For example, the word uninterested may be analyzed as consisting of three morphemes: un-, interest, and -ed. (Morphemes are sometimes also considered to contain units which have no basic phonological representation, see reduplication, truncation).
Morphology - The study of how units of meaning are combined together to create words. The basic units of reference in morphology are morphemes.
Phone - A unit of sound or a gesture used to produce one in a language. Note that different phones may have the same underlying representation in a language (see phoneme).
Phoneme - A class of phones which contrast with other phonemes in a language. For instance, in English the phoneme /t/ is realized as a plosive at the beginning of a word (e.g. term) but as a flap between vowels in most positions (e.g. butter), while in other languages these two sounds are different phonemes.
Phonetics - The study of the different types of sounds used in human language and how they are produced and perceived. The basic units of representation in phonetics are phones.
Phonology - The study of how languages group phones together and use them to encode meaning. The basic units of representation in phonology are phonemes.
Prescriptivism - Ruling on how language should used. This is common in language instruction but is generally irrelevant to linguistics. (Cf. descriptivism)
Register - a different type of speech adopted by a language-user depending on context
Rounding - "puckering" of the lips while producing a phone, most commonly a vowel