Not "atonement," but "satisfaction"
Historical info from a Reformed scholar:
“There has been some scholarly disagreement on this issue -- and sometimes a doctrinal wedge is driven between ‘Calvin’ and the ‘Calvinists,’ as if Calvin taught a ‘universal atonement’ and later Reformed writers taught a ‘limited atonement.’ Yet, when the terms and definitions are rightly sorted out, there is significant continuity in the Reformed tradition on this point.
The terms ‘universal’ and ‘limited atonement’ do not represent the sixteenth-and seventeenth-century Reformed view -- or, for that matter, the view of its opponents. The issue was not over ‘atonement,’ broadly understood, but over ‘satisfaction’ made by Christ for sin -- and the debate was never over whether or not Christ’s satisfaction was limited: all held it to be utterly sufficient to pay the price for all sin and all held it to be effective or efficient only for those who were saved. The question concerned the identity of those who were saved and, therefore, the ground of the limitation -- God’s will or human choice. Thus, both Calvin and Bullinger taught that Christ’s work made full and perfect satisfaction for all, both commended the universal preaching of the Gospel, both taught the efficacy of Christ’s work for the faithful alone -- and both taught that faith is the gift of God, made available to the elect only. In other words, the inference of a limitation of the efficacy of Christ’s satisfaction to the elect alone is found both in Bullinger and in Calvin, despite differences between their formulations of the doctrine of predestination. The Reformed orthodox did teach the doctrine more precisely. In response to Arminius, they brought the traditional formula of sufficiency for all sin and efficiency for the elect alone to the forefront of their definition, where Calvin and Bullinger hardly mention it at all. The orthodox also more clearly connected the doctrine of election to the language of the limitation of the efficacy of Christ’s death, arguing that the divine intention in decreeing the death of Christ was to save only the elect. This solution is presented in the Canons of Dort in concise formula" (Richard A. Muller, After Calvin: Studies in the Development of a Theological Tradition, 14). [SOURCE]