A specific combination of operating system and hardware. Some platforms are:
- Linux on Intel processor ("normal" PC hardware)
- Windows98. We don't have to worry about hardware
- Sun's UNIX OS on the UltraSPARC processor
- Linux on a DEC Alpha processor.
- FreeBSD on Intel
And the list could go on and on....
As you can see though, it is not always enough to specify the OS or the hardware to define a platform. Code compiled on Linux/i386 won't work so hot on Linux/Alpha, or Win98. Now it might work on FreeBSD/i386 or something, but that is because they are being tricky...