Set define meaning in Oracle

Et define off override variable function

The default "&" in Sql*plus represents an alternative variable, that is, Sql*plus will ask you to enter an alternate value whenever the symbol appears in the command. This means that you cannot enter a string containing the symbol into a database or assign to a variable, such as the string "Sql&plus" system will be understood as a string beginning with "SQL", it will prompt you to enter the value of the alternative variable Plus, if you enter ABC, then the final string into " Sqlabc ".
Set define off this function, "&" as a normal character, as in the above example, the final character is "Sql&plus"

Set define off override variable function
Set define on to open alternate variable function
Set define * The default substitution variable marker is "*" (can also be set to other characters)

Set define meaning in Oracle