Search notes:

SQL*Plus: SET ESCAPE

set escape can be used to define an escape character with which the special treatment of the ampersand in substitution variables can be prevented. 
SQL> set verify off
SQL> define val=xyz

SQL> select '&val' v from dual;
V
---
xyz

SQL> select '\&val' v from dual;
V
----
\xyz

SQL> set escape \
SQL> select '\&val' v from dual;
V
----
&val

See also

set
set define off
SQL*Plus

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