Michael E. Fisher, at Cornell University, publishes in 1967 "The Theory of Condensation and the Critical Point", a critical review and extension of the droplet or cluster theory of condensation — the process by which a vapor turns into a liquid. Fisher shows that this theory implies that the condensation point is marked by a singularity of the thermodynamic potential, as Joseph Mayer had conjectured decades earlier. That singularity turns out to be of a particular type — an essential singularity, in which all derivatives of the thermodynamic variables remain finite, unlike an ordinary phase transition. The model also clarifies why a fluid's critical point is unique, in contrast to an extended critical region, and establishes precise relations among the various exponents characterizing the critical point's singularities. Fisher illustrates and confirms his conclusions with an exactly solvable one-dimensional model, with a Hamiltonian containing short-range many-body potentials. The so-called "Fisher droplet model" would become a standard, highly cited tool (over 700 citations) in the study of nucleation and condensation, and would be rediscovered decades later as a key piece in the connection between the Ising model and "Coniglio-Klein clusters" in percolation theory. It is, together with the review and experimental systematization work for which he is more broadly recognized, Fisher's most cited and precisely datable specific theoretical contribution within the body of work rewarded by the 1980 Wolf Prize in Physics.