1. A P-wave, sometimes restricted to P-waves in fluids, but often including those in the solid earth. Synonyms: sound wave, sonic wave. 2. The wavetrain generated and detected by a sonic-logging sonde (see acoustic log and Figure A-4). The wavetrain is a composite of various modes of energy transfer. The first arrival usually results from P-waves traveling in the adjacent formation, where the sonic log measures the slowness (specific transit time or inverse of its velocity). An S-wave traveling in the formation is sometimes a second arrival, but sometimes the second arrival is a tube wave in the borehole fluid. Waves traveling through the mud usually have relatively high-frequency content. Modes of high-amplitude, low-frequency tube waves (sometimes called Stoneley waves) are often distinct arrivals. 3. More generally, any elastic wave or seismic wave.