Sibilance is an exaggerated, harsh-sounding emphasis on 's', 'sh', 'ch', 'z', and 'j' consonants. It occurs when these high-frequency sounds (typically 5–10 kHz) are captured too prominently by the microphone, creating an unpleasant hissing or whistling quality that fatigues listeners.
Several factors cause sibilance: the microphone's frequency response (some mics have a presence boost in the 6–10 kHz range that emphasizes sibilants), the performer's natural voice, and working too close to the mic (proximity effect doesn't cause sibilance but can make an existing sibilance problem more noticeable).
The fix: a de-esser plugin, which is a frequency-dependent compressor that only reduces levels when it detects sibilance in the 5–10 kHz range. Most DAWs include a stock de-esser. Alternatively, positioning the mic slightly off-axis (15–20 degrees) can reduce sibilance capture without a plugin.