carbon-carbon double bond
A carbon-carbon double bond is the C=C link that defines alkenes — two carbon atoms joined not once but twice. Drawn as two parallel lines, it represents four shared electrons rather than the two shared in an ordinary single bond. It is the single most important functional group of this whole field, because almost every reaction here begins with something happening to this bond.
Look closer and the two lines are not equal twins. One is a sigma bond: the carbons' sp2 orbitals point directly at each other and overlap head-on, a strong cylindrical bond like any single bond. The other is a pi bond: each carbon keeps one leftover p orbital standing up perpendicular to the molecular plane, and these two p orbitals overlap sideways, forming an electron cloud above and below the plane. That pi cloud is diffuse and high in energy, so its electrons are loosely held — they sit out in the open where an electron-hungry reagent can reach them.
Two consequences flow from this picture and drive the chemistry. First, the molecule is flat and rigid around the bond: spinning one carbon relative to the other would tear the sideways pi overlap apart, so the double bond cannot rotate, which gives rise to cis/trans (E/Z) isomers. Second, the exposed pi electrons make the bond a nucleophile, attracting electrophiles and launching the addition reactions that make alkenes such rich synthetic raw material.
In ethylene, H2C=CH2, the whole molecule is flat: all six atoms lie in one plane, the H-C-H angles are about 117 degrees, and the pi cloud sits as two lobes above and below that plane, ready to grab a proton or other electrophile.
One sigma plus one pi: the same bond is flat, rigid, and electron-rich all at once.
A common misconception is that a double bond is simply 'a single bond plus another identical single bond'. The second bond is a sideways pi bond, geometrically and energetically different from the head-on sigma bond, and it is the reactive one.