It's mostly just a matter of words here, although physicists tend to
use the words in different contexts so I'll try to explain.
Oscillation, repeating back-and-forth motion, is very common in
nature. Often we think of "Simple Harmonic Oscillation" as a motion
which varies sinusoidally with time:
x = A*cos(omega*t)
for example, where omega and A are just constants, and x is some
variable, such as the position of an object which is oscillating. Many,
many systems oscillate in this way, from a plucked string to a ringing
bell, to radio waves (x is an electric field, say), and lots of other
stuff. Even things that don't move back and forth can oscillate.
Something that gets bigger and smaller (with x being the radius of a
sphere) can be said to oscillate.
"Reciprocation" is most often used to refer to motion that is
repetitive and involves some kind of back-and-forth changing of the
position. I wouldn't demand that the position vary sinusoidally with
time to say that it is reciprocating (although that is nearly the case
with common objects in reciprocating motion). Examples include pistons
in pumps and steam engines on trains as things that reciprocate.
Reciprocating saws have a straight saw blade that is driven up and down
with a motor.
It's also true that oscillators don't have to move sinusoidally
either -- their motion can be described with a sum of sine functions,
or be described as "anharmonic" if it's truly random. Physicists worry
more about oscillating systems of all types and use that word much more
often. You'll find the word "reciprocating" a lot more when you talk
with engineers and people who design things like pumps and saws.
p.s. From what Tom says, it sounds like 'oscillating' may usually
be used to refer to things which go back and forth many times on their
own, like a pendulum or a twangy spring. Even when these things are
driven a little bit, the oscillation rate is mainly determined by the
oscillator. 'Reciprocating' may be used more to refer to things that
are driven back and forth at some rate set more by the driving force,
like the piston in a steam engine.
(republished on 07/13/06)