A syllable is the basic unit of pronunciation: a single vowel sound, optionally surrounded by consonants. Each syllable has exactly one vowel sound (not necessarily one vowel letter). "Cat" is one syllable. "Apple" is two (ap-ple). "Banana" is three (ba-na-na). "Encyclopedia" is six (en-cy-clo-pe-di-a). You can often feel the boundaries by resting a hand under your chin and counting how many times your jaw drops as you say the word aloud.
Syllable counts matter because they feed every major readability formula — Flesch Reading Ease, Flesch–Kincaid, Gunning Fog, and SMOG all use syllables (or polysyllabic word counts) as a proxy for word difficulty. Poets and songwriters count syllables to fit meter, and a haiku's 5-7-5 structure is defined entirely by syllable count.
Counting is deceptively hard. Heuristic counters use vowel-group rules: count runs of vowels as one syllable, then apply corrections such as subtracting a silent trailing "e" ("make" is one syllable, not two) and adding one back for words ending in "-le" after a consonant ("table"). These rules misfire on words like "queue" (one syllable, five vowels) or "area" (three). Precise counts require a pronunciation dictionary such as CMUdict, and even then syllable boundaries can vary by dialect. Our syllable counter estimates counts for whole passages so you can gauge word complexity at a glance.