“Semi-modular” refers to a synthesizer design that combines the pre-configured signal routing found in traditional fixed-architecture synthesizers with the flexibility and patchability of a fully modular system.
A semi-modular synthesizer usually has a pre-wired signal path, meaning that its various components, such as oscillators, filters, envelopes, and modulation sources, are internally connected and can function without the need for additional patch cables. This allows users to create and manipulate sounds quickly, similar to how they would with a non-modular synthesizer.
However, unlike a traditional fixed-architecture synthesizer, a semi-modular synth also provides patch points (usually in the form of 3.5mm jacks or other connectors) that allow users to override the default signal routing and create custom connections between different components. This patchability offers a greater degree of flexibility and experimentation, similar to a fully modular system.