An abstract entity is a conceptual thing. It does not exist as a physical object in space and time, but it can still stem from impressions of the material world. It can be thought about, named, described, and used. Justice is an abstract entity that stems from many impressions of actions, people, situations, and outcomes. So are categories, patterns, and meanings. So are universals like properties and relations. Abstract things include numbers, rights, and possibilities. You cannot pick up justice like a stone, but you can understand it, debate it, apply it, and refine it.
In TST, abstract entities are core to defining the term “idea.” They show how the mind moves beyond direct impressions. The mind can notice a tree and form the category “plant.” It can compare actions and form the idea of fairness. It can observe repeated events and form the idea of cause and effect. These are not concrete objects, but they organize how we understand concrete objects and the world around them. They allow the mind to build ideas from labels, patterns, meanings, judgments, and possibilities.
This is where human thought becomes powerful. Abstract entities let us move from seeing things to understanding and using them. A person sees many individual acts, then forms an idea of morality. A scientist sees many observations, then forms a model. A philosopher sees many experiences, then forms a framework. Abstract entities are not the material world itself, but they are how the mind builds meaning from it.