API

An API (application programming interface) is the contract by which one piece of software exposes functions other software can call automatically: read a calendar, create a customer record, send a text. In practice, when an automatic booking writes into your Google Calendar without you retyping anything, that's an API at work. Most modern so-called REST APIs exchange JSON over HTTPS.

For a solo operator, the stakes are simple: the API is what eliminates double entry. Every piece of information typed twice into two tools is an error source and lost time; every API-based integration removes that cost. Before choosing a tool — CRM, invoicing, telephony — check that it exposes a documented API and native integrations with the rest of your stack.