Elimination reactions remove two atoms or groups from two adjacent carbon atoms and form a brand new double bond between those carbons in their place, structurally the exact opposite of an addition reaction, which breaks an existing double bond open and adds two new atoms or groups onto it. A single-bonded starting molecule can often be pushed toward either outcome, addition-style transformation or elimination-style transformation, and which one actually happens depends heavily on the specific reagents and reaction conditions chosen.
Elimination converts a saturated structure into a double bond by removing, not adding
Where addition takes a molecule with a double bond and saturates it, converting that double bond into a single bond while adding new atoms on, elimination runs in the opposite structural direction entirely, taking a fully single-bonded, saturated molecule and removing two atoms from neighbouring carbons to create a new double bond where none existed before. The two reactions are genuine structural mirror images of each other, one adding atoms while removing a double bond, the other removing atoms while creating one.
Whether a given reaction goes toward addition or elimination is a real, controllable choice
A molecule with a leaving group attached can often be steered toward either an elimination product or a substitution product depending on the specific reagent used and the reaction temperature, since a strong, bulky base tends to favour elimination's double-bond-forming pathway, while a good nucleophile under milder conditions tends to favour substitution instead. This genuine competition between reaction pathways is exactly why organic chemists have to think carefully about reaction conditions, not just about which atoms are technically present, since the same starting material really can be pushed toward meaningfully different products depending on how the reaction is actually run.
What we're still unsure about
That elimination reactions form a new double bond by removing atoms, structurally opposite to addition's double-bond-consuming process, is well established, thoroughly confirmed organic chemistry. What's more genuinely a matter of ongoing predictive difficulty is exactly which specific product an elimination reaction favours when more than one new double bond position is chemically possible, since competing elimination pathways can produce different structural outcomes from the same starting material, and chemists continue to refine and test predictive rules for exactly which pathway dominates under which conditions, rather than one simple rule correctly predicting every case without exception.
This sits inside Elimination Reactions, one of eight topics in Organic Chemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.