A curved piece of wood attached to the bottom of a rocking chair or cradle that enables it to rock back and forth.
Hence, a rocking chair
The lengthwise curvature of a surfboard. (More rocker is a more curved board.)

All modern surfboards share a similar rocker design — Bruce Jones http://www.brucejones.com/longboar.htm
Someone passionate about rock music.
One who rocks something.

curved piece of wood

Hence, a rocking chair

The lengthwise curvature of a surfboard

Someone passionate about rock music

One who rocks something

Extensive Definition

Rocker may refer to:

Rocker (subculture), a British subculture based around motorcycles
Rocker, a person who listens to or makes rock and roll or rock music
Rocker turn, in figure skating
Rocker, a type of furniture, such as a chair or cradle with curved pieces on the bottom, to provide a back and forth motion when pushed or pulled (see rocking chair)
Rocker, a type of cradle used for washing sand or gravel to expose heavier metals, such as gold or silver, used in placer mining

Rocker is also the name of:

Rudolf Rocker (1873-1958), German writer, historian and prominent figure in the international Anarchist movement
Fermin Rocker (1907-2004), painter and illustrator
John Rocker (born October 17, 1974), is a former American Major League Baseball relief pitcher
Lee Rocker (born on Long Island in 1961), is a rockabilly double bass player who started his career with "The Stray Cats"
Tracy Rocker, American college football coach and former player
Rocker, the name of a former drummer of the British indie band The Flatmates

In music

"The Rocker", a song by Thin Lizzy from their 1973 album Vagabonds of the Western World
"The Rocker", a song by Raven from their 1994 album Glow
"Rocker", a 2004 single from Alter Ego
Rocker (song), a song by AC/DC
"Rocker", a song from The 69 Eyes ninth studio album Angels, as well as third single
Rocker, a song by Miles Davis on "''Birth of the Cool" (1950)

In film

rocker in German: Rocker (Begriffsklärung)

Synonyms, Antonyms and Related Words

burin, clarifier, colander, cradle, cribble, die, engraving tool, etching ball, etching ground, etching needle, etching point, filter, graver, intaglio, lixiviator, metronome, needle, oscillator, pendulum, percolator, point, punch, purifier, refiner, refinery, riddle, rocking chair, rocking stone, scorper, screen, seal, seesaw, shuttle, shuttlecock, sieve, sifter, stamp, strainer, style, swing, teeter, teeter-totter, teeterboard, teetery-bender, vibrator, winnow, winnowing basket, winnowing fan, winnowing machine