A job is asking how many years of "professional software development experience" I have.
What does that mean exactly? Any software development done for a job? Or something more strict like using SDLC, git, working in a team, etc?
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It means the length of time that you've been employed as a software developer in an organisation.
It's not a very strict or well defined term, but it's meant to distinguish someone who has spent ten years working full time from someone who has spent ten years tinkering in their free time after work, with the expectation being that the knowledge and experience gained by working full time will be more relevant and useful in a professional capacity than the knowledge gained by working on fun personal projects (not to mention that full time development will involve a lot more hours than doing it as a hobby).
What does that mean exactly? Any software development done for a job?
Literally, yes - any software you've built that somebody else paid you to build.
Or something more strict like using sdlc git, working in team etc
Exactly. The paid (professional) detail is a proxy for ancillary skills and experience on top of fundamental software skills - mostly falling into 3 categories:
As you mentioned, building software as part of a team with all the requisite tooling, process, interpersonal skills, collaboration, etc.
Building software which is fundamentally for other people, and is used by them at times when you're not necessarily on hand to observe, help out or fix issues that come up.
A level of proven general professional competence, orthogonal to software per-se.
It's very hard to gain these ancillary skills from any environment that's not a professional one, which is why they use that term as shorthand.
But any environment where you did gain these skills, a voluntary organisation or large-scale open source project are examples that come to mind, absolutely also 'count' except (perhaps) for point 3.