The use of 'en' to emphasise an infinitive?

Question

So i recently saw the following sentence:

"il faut d'abord bien en définir les termes"

I'm struggling to understand the function of 'en' in this sentence. I asked two native french speakers i know and they both gave different answers

One said that en is being used here in the way that i'm used to. It refers to 'les termes' so the sentence sort of means 'it is first necessary to define them, the terms' with some kind of emphasis on the subject

The second said that en is used to emphasise the infinitive so it could be likened to 'necessary to to define'

neither of these explanations really helped. Under the first, I dont understand why you would use en if you also mentions the actual subject - theres no emission. What does the emphasis mean if that is the case. Usually when something is done for emphasis, the meaning difference is clear with and without the emphasis. But not here.

Under the second explanation, i'm completely lost. what does it even mean to emphasise an infinitive and how is 'en' used to do that?

If anyone can give me more explanation on the structure of this sentence i would be very helpful. Perhaps some examples of other sentences that use 'en' to mean the same thing would help me understand

As a second question, can anyone help me understand how my two native french friends came to different answers to this question? its obviously a pretty difficult abstract concept to describe so it may be a communication issue but neither of them will be 'wrong'. So how do both of these explanations answer the same question in seemingly different ways.

I've asked a number of different questions here but any help on any part at all would be appreciated

thanks

Answer 1

Neither of the explanations provided do work.

En is a pronoun that refers to something that is not in the sentence (similar to of it) so:

Il faut d'abord en définir les termes.

means

"Their terms need to be defined first"

or

"We need to define the terms of it first", i.e.:

Il faut d'abord définir les termes de ce dont nous parlons [ex: du contrat] or whatever this is about.

About your second question. Being a native speaker is not a guarantee of expertise. Of course, having a lifetime of practice in a language helps a lot and gives a strong edge compared to a non native learner, but outside linguists, grammarians and the likes, most of the knowledge average native speakers (including myself) have about their language, whatever it is, is made of habits, intuition, reflexes more than theory. In that case, the intuition of both of your friends misled them although the essential meaning wasn't lost.