The term "pre-established harmony" denotes a predetermined (actually: established in advance) unity.
A basic concept of Leibniz's philosophy , initially to clarify the mind-body problem , then a general expression for the order inherent in all things (unless the causal relationship applies to them), which God has set up in such a way that a harmonious, i.e. parallel, happening takes place, similar to that of two clocks that are precisely regulated ( clock simile ).
Leibniz thus represents a strong ab initio determinism. Since both areas run parallel, it only seems to us that one (e.g. spiritual) area affects the other (e.g. physical). However, this impression of the interaction between substances is wrong and violates the laws of the respective substance area. This also has consequences for the freedom issue at Leibniz.