API
An API (application programming interface) is the defined gateway through which one program reads or changes another program's data.
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Explanation
Through a CRM's REST API, an accounting program or an in-house tool can fetch customers or create a new contact, for example.
An API needs a key; it is good practice to give the key only the permission (scope) it needs.
How HelloTeam does it
HelloTeam's REST API uses scoped keys (read, write, quotes, invoices, form submission), an IP allow-list and a documented OpenAPI description.