I suppose the definition could vary from person to person, but I'd say the key elements would be knowing, and being known by, other forum members. E.g., if I went to a forum you participated in and asked ten very active forum members (or mods), "What do you think about ken_b?", I might get a range of responses:
- "Is he still around?"
- "Sounds familiar."
- "I've seen him around lately."
- "He posts quite a bit, knows his stuff"
- "Good guy. Posts a lot, really knows his industrial widgets. Started a new job last week, so he may be scarce for a while."
The more the answers fall in the last part of the response range, the more I'd say you are a member of the community. Other indicators might be time spent on site, time spent helping other members (as opposed to getting your own issue solved and leaving), inclination to help the forum (by moderating or other volunteer activity), etc.
This is more of a continuum than a cutoff - in any community, be it web or in-person, you have very involved members, not-so-involved members, and entirely uninvolved members.