What does the 's' command in sed stand for?

The 's' command in sed stands for "substitute." It is used to search for a specified pattern in the input text and replace it with a different string. The basic syntax is:

sed 's/pattern/replacement/' file.txt

This command will replace the first occurrence of pattern with replacement in each line of file.txt. You can also use flags to modify its behavior, such as g for global replacement.