Despite its apparent intuitiveness and widespread interest from across various fields, ‘effort’ is a variable that seems difficult to define. The purpose of this article is to consider and define ‘effort’ during task performance. In doing so I argue for a distinction between the actual effort (objective effort) required, and the perception of that effort (subjective effort), during intentional performance of tasks. I adopt a set theoretical approach to defining both actual effort and the perception of effort as both constructs and concepts. Further, I aim to present discussion and definitions that are agnostic of the specific task demands being performed (i.e. physical, cognitive, self-control, or a combination of task demands). Throughout, I attempt to draw upon and synthesise thoughts and ideas from across a multitude of disciplines (though note that this is not intended to be an exhaustive interdisciplinary review), engage in considerable armchair philosophising, and also offer what small insights I have from my own experience both as someone experiencing ‘effort’, and as a third-person observer investigating it. This work is intended to, at the very least, make my own current conceptualisation and understanding of ‘effort’ transparent to other researchers, and aid in the interpretation of any subsequent empirical work on the topic. Further, I hope that it might be of use to researchers from the various fields interested in this topic, and assist in fostering opportunities for integration of learnings across disciplines. It is my intention for this work to support further understanding of the role of ‘effort’ and its perception from a broad scientific perspective.