Oxidation is when oxygen interacts with alcohol, creating degradation of flavour and other chemical reactions

When it comes to spirits, Oxidation is when oxygen interacts with alcohol. In terms of what’s happening, primary alcohols can be oxidized to form aldehydes and carboxylic acids; secondary alcohols can be oxidized to give ketones. Tertiary alcohols, in contrast, cannot be oxidized without breaking the molecule's C–C bonds.

What does this mean for people who don’t wear three-inch-thick nerd glasses? As the booze reacts with oxygen, its flavours diminish over time. If it's in a well-sealed bottle, the effects of oxidation will be very slow unless you agitate it a lot. 

For Gin, oxidation can cause a loss of limonene compounds (citrus), create a gain of carvone compounds (camphor), and the combination of the two means that the effect is more noticeable as it changes the balance of the botanical recipe. You may find that Whisky, Brandy, Rum all taste a little flatter than when it was brand new although, the swing is less noticeable.