Carbonation, also known as crackling, is the process of adding carbon dioxide to cider to produce bubbles. This is common practice in cider making as it is easier and more efficient than bottle conditioning.
Bottle conditioning is accomplished by adding active yeast with a small amount of priming sugar to the cider just before bottling. This process creates natural bubbles without adding carbon dioxide.
So why do cider makers use the term effervescence? Effervescence refers to the cider giving off bubbles of gas. To say cider is effervescent, describes the cider as being bubbly. Like champagne, you can see streams of tiny bubbles moving from the bottom of the glass to the top.
Level of carbonation is used to describe the amount of bubbles and how they taste. Carbonation reduces the availability of free oxygen in cider, and creates a small amount of carbonic acid. This can reduce the pH of the cider by a small amount. Thus it affects taste.
So when I write about effervescence, think streams of tiny bubbles rising in your glass. And when I write about carbonation, think about bubbles that are trapped or suspended in the cider that produce a tingle or sharp taste on your tongue.