This wiki page has the definition of ATC:
Active Technical Contributors ("ATC"),
which are a subset of the Foundation
Individual Members. Individual Members
who committed a change to any of the
official OpenStack projects (as
defined above) over the last two
6-month release cycles are
automatically considered ATC. Specific
contributors who did not have a change
recently accepted in one of the
OpenStack projects but nevertheless
feel their contribution to the
OpenStack project is technical in
nature (bug triagers, technical
documentation writers...) can
exceptionally apply for ATC either by
sending an email to the TC chair or by
being nominated by an existing ATC via
email to the TC chair. Final approval
on the exception is decided by the TC
itself, and is valid one year (two
elections).
ATC can participate to the elections of the Technical Committee members.
In brief, if code (we treat documentation as code) you have authored was merged in any of the current or stable OpenStack release, you're an ATC. Only commits to the official OpenStack programs count to achieve ATC status.
You can verify your most recent merged patches authored by you querying OpenStack review system.