7.28 .equiv symbol, expression

The .equiv directive is like .equ and .set, except that the assembler will signal an error if symbol is already defined. Note a symbol which has been referenced but not actually defined is considered to be undefined.

Except for the contents of the error message, this is roughly equivalent to

.ifdef SYM
.err
.endif
.equ SYM,VAL

plus it protects the symbol from later redefinition.