Is a Middle Name a Given Name?

At least for the Canadian passport application, Given name(s) IS defined as first AND middle names. There is no ‘culture’ argument involved here. However, there are actually some exceptions that would allow for your passport name to not match your DEC (ex. proof of citizenship, birth certificate) as long as it matches another form of ID (ex. driver’s license). It is considered acceptable to drop one or more of your given names from appearing on your passport; If your birth certificate lists your name as Jane Lyn Doe, you may apply for a passport requesting the name Jane Doe OR Lyn Doe, as long as one given name is present. It also doesn’t matter what order your given names come in; If your birth certificate lists your name as Jane Lyn Doe, your passport can have your name as Lyn Jane Doe. You just have to have another form of ID that matches this.

As far as applying for Visas, your name should appear as it does on your passport, so whichever decision you made for your passport, I advise doing the same for your visa. I personally ran into an issue adding my middle name to my visa application when my passport did not have it. Generally you won’t run into an issue doing it the other way around though, dropping given names are more acceptable than adding them.

According to the help file from the Canadian government for filling out the ETA it says the following on the application:

Given name(s) / first name(s)
Please enter exactly as shown on your passport or identity document

Source: https://www.canada.ca/content/dam/ircc/migration/ircc/english/pdf/eta/english.pdf (Page 11)

So you should put it in, exactly like it says on the passport.

If you’re not sure what you should put in (not just two name forms, non-latin script) and your passport has a machine readable zone, you should probably use the names in the same grouping as done there.

The format are two lines at the bottom of the identity page of your passport where the first line is of the following format:
P[4 unimportant letters][family name with blanks and other non-latin characters replaced by <]<<[given names also separated by <] ([] being used by me to separate the fields).

Note that the ETA help pdf has some more detailed explanations that are worth reading for everyone with unusual name(s).

If your first and two middle names are printed under ‘Given names’ in your passport, and you are asked to provide ‘Given name(s)’ not ‘First name’, you should include all three.

Example: I just applied for an ESTA. I have a first and middle name (I’m known by my middle, not first, name), and my passport shows both names under ‘Given names’ and in the machine readable data section. When I uploaded this section during the application process, the system captured both names in the ‘Given names’ section of the application.

Other examples: https://www.canadavisa.com/canada-immigration-discussion-board/threads/is-middle-name-part-of-given-name.202730/

“my First name only into the Given Name(s) field“

If it asked for “first name” a single name might be acceptable. But it didn’t.

Notice the “(s)”. That means that there can be more than one name.

Notice the “Given”, that means all but your family name, which was inherited not given.

The form didn’t ask for “First name”; it asked for “Given name(s)”.

Many countries are improving their forms by saying “Given” rather than “First”, since there are many societies where the first name is actually the family name.
(e.g. in Chinese, “Xi Jinping”, “Xi Jin Ping”, and “Xi Jin-Ping” are all valid Romanizations of “习近平”. “Xi” is first, but it is the family name.)

EDIT: @Voo says that the form contains this instruction:

Please enter exactly as shown on your passport or travel document.

That should remove any confusion. Whatever names are on the passport you are going to be using should be the names you use on the form. No more, no less.

My name is (based on catholic), using the same form as David’s answer:

• A: given (first) name
• B: middle name 1 (or second first name)
• C: middle name 2 (or third first name)
• D: middle name 3 (or fourth first name)
• E: family/surname

To make things even more complicated, my nickname is different than A; it is not in any official paper. So, on my passport I have to use A until D, and not my nickname.

Your title question is answered by your question itself: you consider your middle names as middle names, and not as your first name. Thus, I would complete applications so as to track your passport names as closely as possible.

In my culture, it’d go like this:

• A = given (first) name
• B = middle name #1
• C = middle name #2
• D = family/surname

Like this:

If the application has only two fields, enter A and D in the appropriate fields.

If the application has three fields, enter A in the first field, B and C (with an intervening space if possible) in the second field, and D in the third space.

Different cultures may attribute names differently.