In chapter 18 of 30 Philosophers is this definition.
Universals are the properties, actions, and relations, that are shared by, but may transcend, multiple concrete objects and/or abstract entities.
Notice the use of “but may transcend” in the book. I had to include that broader definition because it was part of the introduction to Plato, his Realm of Forms, and a detailed discussion on universals and the Idea of Ideas. In philosophy, that works. In TST, it is not needed so we have a shorter definition:
A universal is a repeatable pattern among concrete objects, abstract entities, or both.
This definition keeps the useful part and lets go of the extra metaphysical weight. Universals help connect concrete objects and abstract entities in this material world. And, universals can appear in either, or both. The mass of a rock is a concrete object example. Fairness, coherence, and loyalty point toward abstract entities. A butterfly can have physical symmetry in its wings. A geometric shape can have abstract symmetry in math. Same universal, different kinds of entities. In all cases, the mind notices a repeatable pattern. The pattern is what makes the universal useful.
The types of universals are properties, actions, and relations. A property describes what something is like: red, heavy, or tall. An action describes what something does or undergoes: walking, choosing, or promising. A relation describes how one thing stands to another: above, equal to, or caused by. These repeatable features let the mind compare things without treating every moment as completely new.
This also clarifies the difference between universals and schemas. A universal is a repeatable pattern among things. A schema is the mind’s idea of a repeatable pattern. The universal names the repeatable pattern. The mind uses and modifies schemas to make sense of patterns.