A dip oscillator is an oscillator whose oscillation vigor is monitored on a front panel meter. When the exposed inductance of a dip oscillator is brought close to an external tank circuit whose resonant frequency equals the oscillation frequency, that tank circuit draws energy away from the oscillator, diminishing the oscillation's vigor which is then seen as a "dip" in the meter reading.
Going back to vacuum tubes, this kind of device was called a "grid-dip-oscillator" and later, when these things were made using tunnel diodes, there was the "tunnel dipper".
I happen have dip oscillator in the basement, but apart from that, I haven't seen one of these in industry in quite some time. Still, a dip oscillator can be quite handy to have at times.