Joined Feb 20, 2011
what does this mean?

What is the context? Where do you see this filter?

From Wikipedia:

The lower-case letter φ (or often its variant, ϕ) is often used to represent the following:

The upper-case letter Φ is used as a symbol for:

The diameter symbol in engineering, , is often incorrectly referred to as "phi". This symbol is used to indicate the diameter of a circular section; for example, "⌀14" means the diameter of the circle is 14 units.
I would expect in this case it is denoting magnetic flux because the symbol is often paired with a "B" in those cases (though most often it is a subscript):

More wikipedia:

EDIT: Bertus, you beat me to it!
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