Hook
The most memorable phrase, melody, or lyric in a song — the part listeners can't stop singing after one listen.
A hook is whatever burrows into the listener's brain after a single play. It can be a melodic figure (the four notes of "Smoke on the Water"), a lyrical phrase (the "I will always love you" of Dolly Parton/Whitney Houston), a rhythmic pattern (the four-on-the-floor groove of "Billie Jean"), or a production element (the synth sweep of "Take On Me"). Most hit songs have several hooks working together.
The chorus hook is the most important: it usually contains the song title, sits on the strongest melodic figure, and gets repeated enough that the audience leaves humming it. But hooks exist throughout a great song. A good pre-chorus hook ramps tension. A good post-chorus hook lets the listener exhale. A good bridge hook offers contrast. Songs that fail commercially often have a great chorus and weak everything else.
Writing hooks is part craft, part instinct. Test: can you hum it after one listen, without the recording? If yes, you have a hook. If you need to hear it three times, it isn't there yet. Singable melodic intervals (steps, small leaps), memorable rhythms (syncopation that lands on the strong beat), and economical lyrics (three-to-five words, one image) are the building blocks. Strip away until only the unforgettable remains.