This article reports on a study conducted in Croatia on students' understanding of scientific words and representations, as well as everyday words used in chemistry teaching. A total of 82 undergraduate chemistry students and 36 pre-service chemistry teachers from the Faculty of Science, University of Split, were involved. Students' understanding of language was probed using a diagnostic instrument with various types of tasks: creation of a scientifically sensible sentence using the key word provided without context; explanation of the meaning of a word provided in a contextual sentence; selection of the appropriate usage of a term from multiple-choice options; explanation of the meaning of a word provided without context. With every kind of task, evidence of inadequate understanding of many terms and symbols was found. Accordingly, it cannot be presumed that students in Croatia, either undergraduates or graduates, understand well the meanings of scientific words, symbolic representations or everyday words that are used in teaching and learning chemistry. There are considerable differences in the extent of understanding, from word to word, and symbol to symbol. Some of the findings are in common with other studies conducted in English-speaking countries, and some are particular to the Croatian language--especially due to students' confusion in the cases of similar sounding words with different meanings, and the different meanings of words in the everyday and science contexts. Recommendations are made for teaching that involves specific attention to learning about the language associated with topics, through reflective discussion and in formative assessments. Issues of knowledge transfer from research to teachers' pedagogical content knowledge, as well as considerations for further research, are discussed.