What type of word is "as" in "business as usual"?

In the phrase "business as usual", is "as" considered an adverb, a conjunction, or a preposition? (Why?)

Wiktionary on "as" as adverb:

  1. In the manner or role specified.
    The kidnappers released him as agreed.

Wiktionary on "as" as conjunction:

  1. Introducing a basis of comparison, after as, so, or a comparison of equality.
    She's twice as strong as I was two years ago.
    It's not so complicated as I expected.

Wiktionary on "as" as preposition:

  1. Introducing a basis of comparison, with an object in the objective case.
    You are not as tall as me.
    They're big as houses.

As I see the phrase, it's comparing or equating "business" with "usual", which leads me to consider this a conjunction. A friend of mine who I was discussing this with thinks it's more akin to an adverb due to the words it's describing. Maybe it can be considered either or any of these? I'm not a native English speaker and this seems like a grammar technical question that's out of my depth, but I am curious, hence my asking for input here. :)


As can be used in the following ways:

1) as a conjunction (connecting two clauses): As I was leaving, the phone rang. The results were not as bad as I had expected.

2) as a preposition (followed by a noun): He works as a waiter.

3)as an adverb (followed by an adjective, an adverb, or a word such as ‘much’ or ‘many’): Nylon is cheaper than leather, and it’s just as strong.

In this case, "as" in "business as usual" is considered as an adverb. Why? because usual is an adjective.