This report proposes a definition of the concept "will to defend": To have will to defend is to be inclined to act or think in certain ways for a defence-related purpose (similar notions: national will, defence will, willingness to fight, and morale). The definition is explained and justified in more depth in the report. Among other things, in order to enable measurements of the will to defend among the public, a number of operationalisations are proposed, e.g. personal, generic, and supporting will to defend. In light of the proposed definition, it is argued in the report that the will to defend only has an indirect practical significance for the defence capability of a country. What is directly important is rather how people act and what they think, while the will to defend is only one possible motive among others. If the purpose is to persuade people to act and think in certain ways, in order to strengthen the defence capability, other measures might also be needed, in addition to strengthening the will to defend. The report also discusses a number of moral dilemmas. Both the raising of the will to defend and its protection are associated with some difficult-to-solve moral dilemmas.