The term is out of date.
Early computer programs used a line editor program, which evaluated the lines and caught mistakes. The term line editor stands for a copy editor, who very specifically reads a text line by line.
This type of editing requires word for word reading of a text, an excellent grasp of proper grammar, usage and punctuation and is labor intense. It is therefore the biggest ticket item in an editor’s repertoire. It is also the first comprehensive edit I do, and the only one for which you pay me. (I will go through it again before returning it to you. Once you have looked over the edits and made changes, I will do a final proofread.)
The editor may make comments or actual changes to the text to improve readability and change any grammatical mistakes or spelling errors. He or she will also edit for continuity, such as ensuring that the entire piece is written in the same tense (past, present, future) and that if it begins in the first person (I, we) it is consistent, not hopping to second (you) or third (them, they.)
This edit simply will not catch everything. No initial edit ever does.
Your manuscript will require several edits, some say up to seven, and these should not all be done by the same pair of eyes.
Unless you have been picked up by a publishing house, or you have purchased a platinum package from publishers who help you to self-publish, your manuscript will not receive the editing it should. This is reality.
“Indie authors” cannot afford seven edits. This, too, is reality.
I made the executive decision this year to do three edits for the price of one. If I charged for all of them the manuscripts did not come back for another pass under the eagle eye.
If my name will be associated with a piece I want it to be as clean a product as it can be. I don’t have the time to edit something seven times, whether I am being paid or not. And as I mentioned, at least one more pair of eyes should be taking a fresh look at it. However, it is in my best interests to continue to insist on going over it three times. Believe me, I have learned this the hard way.