All assembler directives have names that begin with a period (.). The rest of the name is letters, usually in lower case.
This chapter discusses directives that are available regardless of the target machine configuration for the gnu assembler. Some machine configurations provide additional directives. Chapter 9 Machine Dependent Features.
This directive stops the assembly immediately. It is for compatibility with other assemblers. The original idea was that the assembly language source would be piped into the assembler. If the sender of the source quit, it could use this directive tells as to quit also. One day .abort will not be supported.