A magic string is a string literal (for example, "SELECT", "127.0.0.1") that is used in the middle of a block of code without explanation. It is considered bad practice to use magic strings because:

A string in isolation can be difficult for other programmers to understand.
It can be difficult to update the code if the requirements change. For example, if the magic string represents a protocol, changing the protocol means that you must change every occurrence of the protocol.

Assign the magic string to a new named constant, and use this instead. This overcomes the two problems with magic strings:

A named constant (such as SMTP_HELO) is more easily understood by other programmers.
Using the same named constant in many places makes the code much easier to update if the requirements change, because you have to update the string in only one place.

The following example shows a magic string username. This should be replaced by a new named constant, as shown in the fixed version.

R. C. Martin, Clean Code: A Handbook of Agile Software Craftsmanship, §17.G25. Prentice Hall, 2008.