As a rule of thumb, a "LAN" is a broadcast domain. (It may consist of more than one.) Then again, as a rule of thumb, a rule of thumb applies 3/8ths of the time, so this doesn't really give much to go on.
This was understandably rather poorly received, so I've expanded below.
I've never really liked the essentially recursive answer of "a LAN is a network of devices in the same area." Granted, it's a rather subjective term, so here are a few other options to illustrate what one may be. Note they may not apply in all situations, there may be exceptions... or they may ALL apply to your situation:
Layer 2 Network
If devices are in the same Layer 2 Network, they can be described as being on a LAN.
Devices in a broadcast domain are in a LAN (or multiple LANs). This normally applies to VLANs (Virtual LANs), which can be used to segment a LAN... or can be used to link physically distant devices into essentially the same LAN. Yes, this stretches the definition of "local!"
Non-Routable IP Address
A set of devices on the same non-routable IP address subnet who can talk to each other (without NAT or other workarounds) are most likely on the same LAN as well. What are non-routable IP addresses? Wikipedia explains it well enough: http://en.wikipedia.org/wiki/IP_address#IPv4_private_addresses Essentially, these ranges are set aside for "local" use and are not to be forwarded out to the internet.
These non-routable ranges appear to have been rather arbitrarily chosen--eg, there's nothing special in electricity that makes 10.0.0.0/8 range not route, but someone realized some ranges should be set aside and made a judgement call about which ones. I could be wrong (which isn't rare by a long shot), but Jodies.de lets us see the binary of these ranges ( http://jodies.de/ipcalc?host=10.0.0.0&mask1=8&mask2= ). I don't see anything terribly remarkable about the addresses. They just seem arbitrary. See for yourself:
Address: 10.0.0.0 00001010 .00000000.00000000.00000000
Netmask: 255.0.0.0 = 8 11111111 .00000000.00000000.00000000
Address: 172.16.0.0 10101100.0001 0000.00000000.00000000
Netmask: 255.240.0.0 = 12 11111111.1111 0000.00000000.00000000
Address: 192.168.0.0 11000000.10101000 .00000000.00000000
Netmask: 255.255.0.0 = 16 11111111.11111111 .00000000.00000000
Additionally, a VPN tunnel (be it just from your PC to your company or a site-to-site/LAN-to-LAN tunnel) lets a remote machine behave as though it were on the LAN. If it walks like a duck, and it talks like a duck, and it tastes like a duck...
"LAN" and "WAN" are rather ambiguously defined. They are hardly the only delineations, either--I've been trained on a model that went from LAN to optional CAN (Campus Area Network) and/or to MAN (Metropolitan Area Network) to WAN. You may define your LAN as everything behind your home internet modem, but your ISP could consider every subscriber on your block to be in a LAN... and they could both be right simultaneously! The terms can subjective to a degree, but these guidelines (and all the others posted) should help you determine if something is a member of a LAN, a WAN, or both.