Repeating one block of text across multiple posts or pages

In the Admin Panel I’d like to create a block of text, like this:

code: [[text-block1]]
message: “Hello world!”

… and in multiple posts or pages if I type in [[text-block1]], they all will show the same message “Hello world!”

If I need to change “Hello world!”, I only have to do it in one location (associated with [[text-block1]]) and the updated message will be reflected in every post or page. Sounds extremely simple, right? The problem is, I don’t know how to do this. The “Widget” almost seems to be able to do this, but it’s restricted to right or left side columns. I need to be able to insert a pre-defined block of text WITHIN a post or page.

I looked all over for a plugin that supports this, to no avail.. please help. Or should I have to make the plugin myself?

Thanks,