The aim of this paper is to establish conceptualization and classification of reduction that can better capture the gradual form reduction phenomenon by re-examining concept of abbreviation based on ‘The Korean Corpus of Spontaneous Speech’. The concept of reduction modified by re-examining abbreviation is gradual form erode phenomenon of acoustic and phonological dimension, which is not always correlate with semantic changing or grammaticalization. Although aspect of reduction has general phonological tendency, this phenomenon is very different for each word. Reduction is classified with acoustic reduction and phonological reduction based on whether or not a segmental sound domains. Acoustic reduction is erode of non-segmental sound dimentions such as the temporal domains or spectral domains. Phonological reduction is erode of segmental sound dimentions that is subclassifed as a feature reduction, phoneme reduction and syllable reduction.