do mirrors reflect all frequencies of light or just the visible spectrum?

Light is, by definition, only what you can see. However, if you are
asking what spectra mirrors can reflect, I expect a Researcher will
come up with a very satisfactory answer.

But let me give you a hint-- ever tented a turkey?

The simple answer is no and no. No mirror reflects all frequencies of
light, but most mirrors work quite well at some nonvisible
wavelengths. For example a thick, polished piece of metal works well
at very low frequencies and at visible wavelengths, but stops working
at very high frequencies. A dielectric interference mirror typically
works over a very narrow range of wavelengths. Some mirrors, like
those used for Xrays, may work only over a very restricted range of
angles. It all depends.

To comment on the earlier comment, the word light is often used to
refer to other parts of the spectrum. For example, there is infrared
and ultraviolet light. Commonly the term is used generically to refer
to the entire spectrum of electromagnetic waves.

i don't know what turkeys and tents have to do with anything but
kottekoe provided a very reasonable answer and richly deserves the
princely sum of 2 dollars.

The first definition of light is: Electromagnetic radiation of a
wavelength in the range from approximately 4,000 (violet) to about
7,700 (red) angstroms and that can be perceived by the unaided human
eye.

A lesser definition is EM radiation of any wavelength.

Calling this secondary definition "common" borders on disingenuous.
Because a definition enjoys prominence in a small community-- say,
physicists-- does not give license to extrapolate that usage.

As for turkey tenting; you tent a turkey with foil to reflect heat
(infrared) back to the turkey. The actual mechanism is a little more
complicated but that is part of the reason.

It is interesting to note that the definitions for "mirror" includes
references to reflecting light and forming images while the definition
for "reflector" specifically makes mention of other spectra (light,
heat, sound, radiation). So a mirror is a reflector, but a reflector
isn't necessarily a mirror.

For my next trick I shall prove that God exists, which will mean he
doesn't (Adams, 1978).

Tch, tch Mark.  You're getting touchy.

I think "ultraviolet light" is a very common, well-known, and
well-understood phrase, even though it refers to an invisible form of
light.

If we are being picky, let me suggest we not confuse infrared
radiation and heat. This confused the heck out of me as a child.

For the purpose of answering this question, I'll use the broadest
definition of light, i.e. all electro-magnetic (EM) radiation.
I'll also use the word mirror to mean "EM reflector".

Kottekoe's answer is a good one. I could also add that there are
reflectors (call them mirrors if you want) that are specifically
designed to pass some wavelengths while reflecting others - these
are known as dichroic filters (as opposed to absorptive filters).

Examples are "hot" and "cold" mirrors. Hot mirrors reflect infrared (IR)
while letting visible light pass right through. Cold mirrors do the
opposite.

Of course, mirrors don't have to be flat. You can find a parabolic
cold mirror in a halogen spotlight - the IR goes out in every direction,
while the visible light is focused into a beam.

Some video projectors contain dichroic filters that reflect some visible
wavelengths while reflecting others. These are used to add colour to
display technologies which would otherwise be monochrome.

A mirror can have holes in it and still work, provided that the holes are
much smaller than the wavelength of the radiation to be reflected. The
perforated metal gauze in a satellite dish or in the door of a microwave oven
reflects microwaves (wavelength around 2cm) while letting at least some
visible light through.

Arguably, dichroics should really be called "filters", rather than "mirrors",
but terms like hot and cold mirror still persist.

How about some real data.
http://www.edmundoptics.com/TechSupport/DisplayArticle.cfm?articleid=259
Remember, aside from front surface mirror for scientific purposes, you
need to account for the "glass" in front of the mirror.

i'd like to add that turkeys look funny in the mirror.