allylic system
/ uh-LIL-ik /
Stand next to a double bond and look at the carbon just beyond it — the saturated carbon directly attached to a C=C. That neighbouring position is called the allylic position, and any group of three carbons built around it, CH2=CH-CH2-, is the allyl system. The name comes from allyl, the CH2=CH-CH2 fragment found in garlic oil; the key idea is that being right next to a double bond makes this position special.
What makes it special is conjugation through a single empty, half-filled, or filled p-orbital. If a charge, a radical, or an electron pair lands on the allylic carbon, that carbon can rehybridize to sp2 and offer up a p-orbital, which then overlaps with the adjacent double bond's pi system. The result is a three-carbon, delocalized pi system in which the reactive centre is shared over the two end carbons instead of being stuck on one. You can draw it as two resonance structures with the double bond and the charge/electron swapping ends — but remember those are two pictures of one delocalized hybrid, not a molecule flickering between forms.
The allylic system matters because that delocalization is real stabilization, and it changes outcomes. Allylic cations, radicals, and anions are all noticeably more stable than their ordinary (non-allylic) counterparts, which speeds up reactions that form them and explains why allylic positions are favoured sites for radical halogenation (think NBS) and why allylic substrates can react by special pathways. Whenever a reactive intermediate can spill its instability into a neighbouring pi bond, chemistry gets easier there.
In propene, CH2=CH-CH3, the CH3 is the allylic position; pulling a hydrogen off it makes the allyl radical CH2=CH-CH2 dot, whose unpaired electron is shared over both end carbons and which is therefore far more stable than a typical primary radical.
The carbon next to a C=C is the allylic position; a charge or radical there delocalizes over three carbons.
Allylic means the carbon next to (adjacent to) the double bond, not a carbon of the double bond itself — that is the vinylic position; mixing up allylic and vinylic reverses much of the reactivity story.