Critical point is the transition temperature of a second order phase transition. But what does tricritical point mean? WIki says that a tricritical point is a point in the phase diagram of a system at which three-phase coexistence terminates. But in this paper, it seems that the meaning of tricrital is different from Wiki.
There is no contradiction between the explanation of the tricritical point in the Wikipedia article and the usage of the term in the paper. In the latter, they use a model which consists of several components and exhibits phase transitions. They identify a tricritical point which fits the definition in the Wikipedia article.