Deep tech is the darkened, bass-heavy house sub genre pioneered by London DJs like Mark Radford (check the output of his Audio Rehab label for examples). Its become prominent in the UK capital and beyond and got its name after Radford kept having to explain to people the kind of music he'd play in his DJ sets and on his Rinse show. "I alway say it's a cross between deep house and tech, so deep tech was the tag that everyone starting giving it," he says. Simple, and effective.
According to this great Ron Trent interview, "deep house" was the name for "classics, disco and obscure music" played out at legendary clubs like Shelter, The Loft and Paradise Garage back in the day. He reckons MFSB's 'Love Is The Message' is the "Godfather" track of deep house, calling it "tasteful and timeless".