'Tropism' means leaning toward or away from a stimulus; 'mono' of course means one. Literally, monotropism means leaning toward (or away from) a single stimulus.
Dinah Murray, Mike Lesser, and Wendy Lawson suggest that people exist on a continuum ranging from polytropic (having many diffuse interests) to monotropism (having interests that are few, but intense). Those particular theorists posit that autistism occurs at the most montropic/attention-channeled end of the continuum.
I'm not autistic, but I do have monotropic...well, leanings. The Little Prince is a story the monotropic within me understands well. Of course the rose that the Little Prince has invested his time and tears in is unique in all the world (even if an ordinary passer-by thought it looked identical to 500 other roses). And of course one can find in a single rose or a single drop of water what they can't find in a garden or a well.