chirality center
Picture the spot in a molecule that acts like the hub of a tiny three-dimensional pinwheel — change the order of the blades and you get a genuinely different shape. In most everyday organic molecules that hub is a single carbon atom, and it is the place where 'handedness' is born. We call it a chirality center.
Formally, a chirality center is an atom holding four different groups arranged tetrahedrally, so that swapping any two of them produces a stereoisomer rather than the same molecule. For carbon this is the familiar asymmetric carbon: bonds point to the four corners of a tetrahedron, and four distinct substituents make the two possible mirror arrangements non-superimposable. To spot one, find a carbon and trace the four groups attached; if all four are different (even if the difference lies several atoms away down two chains), it is a stereocenter. A carbon with two identical groups, a double bond, or only three groups is not.
Counting stereocenters is the first move in any stereochemical analysis, because it tells you how many stereoisomers to expect (up to 2^n for n centers) and where the molecule's three-dimensional decisions live. Most chiral molecules of biology — sugars, amino acids, steroids — are studded with several stereocenters, and naming the configuration at each (R or S) is how chemists pin down exactly which isomer they hold.
In 2-butanol, CH3-CH(OH)-CH2-CH3, the second carbon carries H, OH, CH3, and CH2CH3 — four different groups, so it is a chirality center. 2-propanol, CH3-CH(OH)-CH3, has two identical CH3 groups on the middle carbon and therefore has none.
Four different groups make a stereocenter; two identical groups destroy it.
Common misconception: having a stereocenter does not automatically make a molecule chiral. A meso compound has stereocenters yet is achiral overall because of internal symmetry. 'Stereocenter present' and 'molecule is chiral' are related but not the same claim.