1. Applying to some but not to others.
2. Tending to make careful choices.

1. A circuit that passes selected frequencies of alternating currents while weakening other frequencies.
2. A network that transmits alternating currents of desired frequencies while substantially attenuating all other frequencies.

1. A chemical transducer that yields a response to variations in the concentration of a given ion in solution.
2. A potentiometric electrode (electromotive force or pressure in an electric circuit measured in volts) that develops a potential in the presence of one ion (or class of ions), but not in the presence of a similar concentration of other ions.

A field-effect transistor whose gate electrode is sensitive to certain ions in an electrolyte, so that the gain of the transistor depends on the concentration of these ions.

A seemingly incongruous behavior in which someone exhibits a ready flow of speech in private, but silence in public: Little Patrick can talk with members of his family; however, he never says anything when he is at school.