Business operating system
A business operating system is the one place where a small business's goals, customers, work and records live together, so that deciding what to do next does not require assembling the picture from six separate tools first. It is a way of running the business rather than a category of software feature.
Why the phrase exists
Most small businesses end up with a booking tool, a mailing tool, a spreadsheet of customers and a folder of invoices, each holding one true fragment of the business. Nothing is wrong with any of them individually, and together they make a simple question — is this month better or worse than last, and because of what? — expensive to answer. A business operating system is the response to that: one place the fragments agree.
How it differs from a CRM or a project tool
A CRM knows about customers and a project tool knows about tasks. An operating system is the layer that connects them to a goal: it can tell you that a follow-up did not happen, that the reason matters, and what it costs. The distinction is not features but whether the system can reason about the business as a whole rather than about one object type at a time.