Sentence:  The identification of the basic <WSD>unit</WSD> of religious organization - the parish or congregation - with a residential area is self defeating in a modern metropolis , for it simply means the closing of an iron trap on the outreach of the Christian fellowship and the transmutation of mission to co-optation . 
Meaning of the word marked with <WSD> tags:  a single undivided natural thing occurring in the composition of something else