Growing up as I did in the ancient days before either home video or cable TV were ubiquitous, my first encounters with cinematic sci-fi was watching old films on Saturday afternoons. A lot of these films weren't particularly good, but many of them, like The Day the Earth Stood Still and Forbidden Planet to mention just two, were and they played an incalculable role in forming my imagination. Even now, more than three decades later, the influence of those SF movies remains powerful, such as my fondness for the theremin.
The theremin, in case you've never heard the word before, is an electronic musical instrument invented in 1928 by the Russian inventor Lev Termen, known in the West as Léon Theremin, after whom the instrument is named. Here's a video of the man himself demonstrating its use:
During the 1950s, the theremin was used a great deal -- some would say overused -- in science fiction films, which is why I so strongly associate its distinctive sound with the genre. It definitely has an otherworldly quality to it that conjures up alien vistas for me. I understand that to many other people, the theremin sound represents cheesy sci-fi and so they abominate its use. I can understand that. Watch enough Ed Wood-level schlock and there's a good chance you'll hate the theremin, too. Not me, though. In fact, I'd like to hear it used more often.
(As an aside, it's worth mentioning that the soundtrack to Forbidden Planet did not make use of the theremin. Instead, composers Louis and Bebe Barron made that movie's music completely electronically, which is to say, without the use any instruments beyond electronic circuits, whose noises they then manipulated by playing them backward and forward and otherwise modulating them to produce the desired sounds. Considering that they did this nearly a decade before the invention of the first synthesizer, it's a pretty impressive accomplishment.)