The default Ant build file is named "build.xml". Any build system beyond the most basic will have multiple build files for various modules of the application. Having them all named "build.xml" quickly leads to confusion of which does what.
Build file names should indicate the main purpose of the file, and should be in the Java style of mixed upper and lower case letters, with the first letter always capitalized. Words should be spelled out rather than abbreviated, unless the abbreviation is the more common reference. Using the main target name works well.
Build files may load properties from a properties file. These property files should be named the same as the build file name, but with a ".properties" extension.