A low-profile microphone with a semi-cardioid pick up. Most commonly used in theatrical productions. Good for picking up chorus and general ambience, and especially effective when there is a proscenium arch between the cast and the audience.
It's generally best to space a few of these across your stage, as a rough rule of thumb the distance between the mics should be a minimum of three times the distance between mic and cast members.
Boundary mics are great for picking up a chorus or entire cast. Individual voices are a bit trickier, especially if they are quietly spoken. Most successful productions use radio microphones on leading cast members and use boundary mics to pick up the rest.