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Checkin means the action of checking into a location and sharing that information. The action itself is often referred to as a checkin.
Services like Foursquare, Gowalla (shutdown after being acquired by Facebook), Dodgeball (shutdown after being acquired by Google), Google Latitude (also shutdown by Google) are social community sites specifically designed for sharing checkins.
The indieweb community is exploring how to post checkins to one's own website (likely privately), and how/when to syndicate (in POSSE style) such checkins to such social community sites.
Past discussion sessions on indieweb checkins:
Pretty good notes in there that could be abstracted and captured in general form in new sections here.
I'm entering a specific venue, I'd like to quickly/automatically capture the fact that I'm entering that venue (similarly for exiting) for personal logging / quantified self purposes.
I'm visiting a city other than my home town, and rather than try to figure out who I know may be in that town at the moment and compile far too many explicit messages or a long BCC email, I'd like to announce to my friends who happen to also be in that city that I've arrived there and will be around a bit in case they'd like to meet up.
I'm out at a coffee shop and want to quickly see if any friends are are nearby and might interested in getting brunch/lunch/dinner/drinks together.
In rough launch/implementation order:
Barnaby Walters is using Taproot on waterpigs.co.uk to post notes with location data (including rev-geocoded address data) represented with an embedded h-card venue and in an embedded map, since 2013-01-24. Currently manually tagging specific check-ins with #checkin, but is working on adding richer venue information. Examples:
More ideas than creations, some thoughts about how individual indieweb community folks want to do indieweb checkins.
Main article: venues.
I'd rather post a check-in on my own site, and have it use a venue URL that is *also* on my own site. Then at that indieweb venue URL, it can link to equivalents on Foursquare, OSM, FB, and whatever other centralized venue databases are created in the future. - Tantek 
Experience with Foursquare's "community generated/patrolled venues database" is that a few nitwits can pollute/damage the data quite badly, with deletions, overmergings etc. Sad to say, same thing happens with Wikipedia (excessive deletions, overmergings). Thus little hope that a "community hosted venues database" wouldn't have all the same problems, if not worse.
Just use hCard on indieweb venue URLs.
In general it is good to use multiple source of geolocation information in order to increase accuracy and decrease latency to provide this information when querying for and presenting (multi-factor prompts) nearby venues.
GPS is great. Except when you are inside a building - it doesn't work. Then you go and get on the Tube, and pop out the other side. It takes time for GPS to seek. You spend much more time waiting for GPS to work out where you are than you do just typing in the name of the bar or whatnot.
wifi-assisted location determination can do better than GPS when inside a building.
Last time(s) you checked in (or GPS was detected), you were moving in this direction at this pace, therefore you're likely at this location now.
The last n times you checked in at place A, you checked in at place B within the next thirty minutes.
You just checked in with friend F at location A, your friend F just checked in at location B, therefore you might be at location B as well.