isolated diene
An isolated diene is a molecule with two carbon-carbon double bonds that keep to themselves — they are far enough apart that they neither overlap nor talk to each other. Between the two double bonds sits at least one sp3 carbon (a CH2 or similar saturated centre), which acts like a wall: each double bond behaves like an ordinary, independent alkene, as if the other were in a different room.
The defining feature is that single, saturated insulator. In 1,4-pentadiene, CH2=CH-CH2-CH=CH2, the central carbon is sp3 with no p-orbital to share, so the two pi systems cannot line up and merge. Without continuous p-orbital overlap there is no delocalization and no conjugation bonus — the molecule's energy is essentially just the sum of two separate double bonds. Chemists sometimes call these non-conjugated or, when the gap is exactly one CH2, 1,4-dienes.
Why bother naming them? Because comparing isolated, conjugated, and cumulated dienes is the cleanest way to see what conjugation actually buys you. An isolated diene is the baseline: take it as 'two normal alkenes,' and a conjugated diene comes out measurably more stable while a cumulated diene (allene) comes out less stable. Isolated dienes react chemically just like simple alkenes at each double bond independently, and they cannot do a Diels-Alder reaction, because that demands the two double bonds be conjugated and able to swing into the s-cis shape.
1,4-Pentadiene (CH2=CH-CH2-CH=CH2) has its two double bonds insulated by a central sp3 CH2, so each behaves as an ordinary alkene and the molecule gains no conjugation stabilization.
A saturated carbon between the double bonds blocks p-orbital overlap, so there is no delocalization.
Do not mistake an isolated diene's lack of conjugation for instability — it is perfectly stable, just not extra-stabilized; it is the cumulated diene (allene), with two double bonds crammed onto one carbon, that is the strained, less-stable arrangement.