The article deals with the semantic career of virtus as a political concept in the Middle Ages. It traces the different aspects of meaning assigned to this word in four medieval texts, namely St. Augustine's City of God, the Regula Pastoralis of Gregory the Great, the Via Regia of Smaragdus of St. Mihiel, and the Policraticus of John of Salisbury. Using quantitative methods, I analyze the employment of virtus with a focus on its relevance in the political discourse, and I also address the shift in meaning and argumentative capacity that the term undergoes over time. In the end, virtus can be shown to be a highly flexible yet strongly functional term that plays an important role in the conceptions of medieval societies.