A Wikipedia definition is:
Holism (from ὅλος holos, a Greek word meaning all, entire, total) is the idea that all the properties of a given system (biological, chemical, social, economic, mental, linguistic, etc.) cannot be determined or explained by its component parts alone. Instead, the system as a whole determines in an important way how the parts behave.
In other words, Holism is a property of any System. But what is Systems? Wikipedia says:
System is a set of interacting or interdependent entities, real or abstract, forming an integrated whole.
Then, Holism could be used as an approach to understand the reality or abstraction. No?. Yes!.
A map-printed is a system, in this case it represents a specific geography/location (reality). Its elements or parts are: paths, orientation, size/scale, blocks, streets, traffic signs, etc.
Commonly, this map-printed is called model.
A PC is other example of system. In Software Engineering a Database Server is a set of elements as PC-server, DDBB engine, tables, rows, columns, metadata, data, constrains, etc. A representation of the Database Server could be a Entity-Relation model.
The Global warming Problem is other kind of System and it is possible to get a model too . It is a complex problem because is a big-System with many element and unknow relationships beetwen its parts. There is not enough paper for drawing this model.
Well, Holism is an approach that help us to interpret a System.
Here a table that summarize all that a bit more clearly.