Critical citations are lacking a common definition in relation to extant research on knowledge construction and citation analysis, whereas studies on these topics seem to provide a fully relevant theoretical framework, making criticism an essential phenomenon for the progress of science. We propose to explain this paradox by the fact that a citation seems to have a positive polarity by default and that a polarity shift is the result of a stronger commitment on the part of the author. This results in the use of specific cue words. By studying the labels (and their associated definitions) that 53 other studies equated with the concept of critical citation, we identified 3 functions on which to base the definition of critical citation: "to criticize", "to compare" and "to question" other works. While these studies seem to consider the criticize function as central and probably more frequent, the analysis of a corpus of 51 text snippets containing a citation (all retrieved from those same studies) reveals that the citations considered as critical by these same authors are often comparisons between results rather than blunt attacks against the cited works. This three-function based definition and the set of wordings gathered in this study provide a new basis for the design of tools dedicated to citation polarity detection. Indeed, the lexical and grammatical markers characterizing comparison must be taken into account in addition to those expressing a negative evaluation and those expressing doubt.