I am just wondering why some electret mics are called omnidirectional. Does this just mean the source of sound doesn't have to be straight along the axis of the mic, and it will still pick it up well?

 

They simply have the diaphragm as one surface of an otherwise sealed capsule.
Air pressure changes from sound waves will be sensed to some extent pretty much regardless of direction.

Directional electret inserts typically have a ring of vent holes behind the diaphragm, plus internal vents in the diaphragm mounting plate, allowing pressure to reach both sides of it.

 

I am just wondering why some electret mics are called omnidirectional. Does this just mean the source of sound doesn't have to be straight along the axis of the mic, and it will still pick it up well?

here is what wikipedia says....

An omnidirectional (or nondirectional) microphone's response is generally considered to be a perfect sphere in three dimensions. In the real world, this is not the case. As with directional microphones, the polar pattern for an "omnidirectional" microphone is a function of frequency. The body of the microphone is not infinitely small and, as a consequence, it tends to get in its own way with respect to sounds arriving from the rear, causing a slight flattening of the polar response. This flattening increases as the diameter of the microphone (assuming it's cylindrical) reaches the wavelength of the frequency in question. Therefore, the smallest diameter microphone will give the best omnidirectional characteristics at high frequencies.
The wavelength of sound at 10 kHz is little over an inch (3.4 cm) so the smallest measuring microphones are often 1/4" (6 mm) in diameter, which practically eliminates directionality even up to the highest frequencies. Omnidirectional microphones, unlike cardioids, do not employ resonant cavities as delays, and so can be considered the "purest" microphones in terms of low coloration; they add very little to the original sound. Being pressure-sensitive they can also have a very flat low-frequency response down to 20 Hz or below. Pressure-sensitive microphones also respond much less to wind noise than directional (velocity sensitive) microphones.