Seen it referred to on some other threads and its referred to in a negative way.
DS 2.5 is really into his letters at the minute so we have to play bath letters in bath every night (sometimes i say shall we play with the ducks? reply' no letters'. Today we played playdough with cutters in the shape of letters, he likes playing with his magnetic letters on his easel all the time, when we go out for a walk or to the park we have to stop at every road sign and he likes to name the letters
Sounds like he's initiating his letter learning, not you pushing him into it.
To my knowledge 'hot housing', (referring to tropical plants grown in green houses in climates they wouldn't otherwise naturally thrive) is usually the parent pushing/forcing the child to learn things.
My Dd was very into numbers and letters from the word go and I think she just likes stimulation. I didn't push her into it (no spectacular aptitude for it myself) she just wanted to do it. Gets it from DH, I think.