A model is “open weights” when the organization that trained it publishes the trained parameters - the actual numbers that make up the model - so anyone can download, run, fine-tune, and deploy it on their own hardware. A “closed” model, by contrast, lives behind a vendor’s API: you send it inputs and get outputs, but you never possess the model itself. The difference shapes cost, control, privacy, and how much you depend on a single provider.
A common confusion is treating “open weights” as the same thing as “open source.” The Open Source Initiative, which has long defined what “open source” means for software, published an Open Source AI Definition that sets a high bar: a genuinely open source AI system must grant the freedoms to use, study, modify, and share it, and must provide not just the parameters but also “the complete source code used to train and run the system” and “sufficiently detailed information about the data used to train the system so that a skilled person can build a substantially equivalent system.” Most widely used “open” models do not meet that full bar, because they release weights without fully disclosing training data and code.
The contest is sharpest around licensing. Meta’s Llama models, for example, are distributed under a custom community license, not an OSI-approved open source license: it restricts very large companies (those above a stated monthly-active-user threshold) from using the models without separate permission, and forbids using the outputs to improve competing large language models. The weights are downloadable, but the terms attach conditions that standard open source licenses do not. This is why careful writers say “open weights” rather than “open source” for such releases.
Why business readers should care: the choice between open-weights and closed models is a strategic one. Open-weights models offer control, on-premises privacy, and freedom from per-call pricing, but require you to run the infrastructure and to read the license carefully - “open” does not automatically mean “unrestricted commercial use.” Closed models trade that control for convenience and a managed service.