Ambiguity of language can form a barrier to understanding mathematical concepts. The word 'capacity' is problematic as it holds a variety of meanings and might not be understood in school mathematics, to be an aspect of volume. We investigated pre-service primary school teachers' interpretations of 'capacity' when given the definitions of 'volume' and 'capacity' provided in the Australian Curriculum: Mathematics. Of the 72 participants, 20 appeared to hold the conception that the volume of a liquid is its capacity. We suggest reducing ambiguity by defining 'capacity' more clearly or by excluding this word from formal mathematics terminology.