Yes, it is injection because you are injecting your code into the existing SQL code.
For example. A common way to log a user in using php and SQL looks like this:
$sql = "SELECT * FROM users WHERE username = '" . $POST['username'] . "' AND password = '" . $POST['password'] . "';"
The variables $POST['username'] and $POST['password'] what the user puts in the login fields. If the user entered the following.
' or 1 == 1 ---
The resulting SQL that is executed looks like this:
SELECT * FROM users WHERE username = '' or 1 == 1 --- AND password = '';
Notice that everything after the "---" is a comment and ignored. So the sql looks to see if it can find a username that matches null or 1 == 1. Since 1 always equals 1, the SQL will return true and log the user in.
This is called injection because the user is injecting their code into the code the developer wrote.