Platform Defines Macros

The platform defines that Unity supports for your scripts are:

UNITY_EDITOR	Define for calling Unity Editor scripts from your game code.
UNITY_STANDALONE_OSX	Platform define for compiling/executing code specifically for Mac OS (This includes Universal, PPC and Intel architectures).
UNITY_DASHBOARD_WIDGET	Platform define when creating code for Mac OS dashboard widgets.
UNITY_STANDALONE_WIN	Use this when you want to compile/execute code for Windows stand alone applications.
UNITY_STANDALONE_LINUX	Use this when you want to compile/execute code for Linux stand alone applications.
UNITY_WEBPLAYER	Platform define for web player content (this includes Windows and Mac Web player executables).
UNITY_WII	Platform define for compiling/executing code for the Wii console.
UNITY_IPHONE	Platform define for compiling/executing code for the iPhone platform.
UNITY_ANDROID	Platform define for the Android platform.
UNITY_PS3	Platform define for running PlayStation 3 code.
UNITY_XBOX360	Platform define for executing Xbox 360 code.
UNITY_NACL	Platform define when compiling code for Google native client (this will be set additionally to UNITY_WEBPLAYER).
UNITY_FLASH	Platform define when compiling code for Adobe Flash.

Also you can compile code selectively depending on the version of the engine you are working on. Currently the supported ones are:

UNITY_2_6	Platform define for the major version of Unity 2.6.
UNITY_2_6_1	Platform define for specific version 1 from the major release 2.6.
UNITY_3_0	Platform define for the major version of Unity 3.0.
UNITY_3_0_0	Platform define for the specific version 0 of Unity 3.0.
UNITY_3_1	Platform define for major version of Unity 3.1.
UNITY_3_2	Platform define for major version of Unity 3.2.
UNITY_3_3	Platform define for major version of Unity 3.3.
UNITY_3_4	Platform define for major version of Unity 3.4.
UNITY_3_5	Platform define for major version of Unity 3.5.
UNITY_4_0	Platform define for major version of Unity 4.0.