The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know and can’t guess what word you wanted or what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you a pass anyway.
Autocorrect suggests words that start with the same letters. It’s suggesting what word you may want to save time, but quite often, its suggestions are pretty off base. They don’t help you out, but they do make you laugh.
Theorem is a noun. It’s pronounced “th-eer-em.” It means a proposition or formula based on other propositions or formulas, that needs to be proven. Theorem is related to the word theory. In logic, it means an idea that seems legitimate, but hasn’t been tested or proven for confirmation.
Theremin is a noun. It’s pronounced “th-air-em-en.” It means a musical device created in 1920s by Leon Theremin. It has a large antenna-like rod pointing upward on one side, and a small hoop antenna pointing outward from the opposite side. Both antennae are mounted on a small box with dials on it.
Unlike many traditional instruments played with the mouth or touched with tactile keys or buttons, the theremin is “played” by moving one’s hands in proximity to the antenna. Here’s a video of its creator playing it.
Its sound, to me, resembles the higher notes of a violin, but with more tremolo. In US film and TV, the violin is used to lend sad, melancholy, contemplative, or sleepy mood to the story, and the theremin is used in a similar fashion. Perhaps because of its unique appearance and playing style, the theremin has been used historically in horror and sci-fi films.
Clara Rockmore, a legendary theremin performer, was recognized with a google doodle in early 2016.
The following story uses both words correctly:
Thais got into college on a theremin scholarship. Upon graduation, she taught algebra and geometry at an esteemed private school in the city. Her students appreciated the way she made numbers and theorems engaging and fun. She was credited with inspiring more young women to pursue STEM careers.