“Temptation—they always speak with heavy emphasis: Great Temptation. You see, we who employ the Bene Gesserit teachings can influence such things as the internal adjustment of enzyme balance within our bodies. It can prolong youth—far longer than with melange.” - Children of Dune
Great Temptation refers to the temptation faced by a Bene Gesserit trained adept to alter their internal chemistry in order to extend their youth and longevity far beyond that which melange was capable of. As the spice was capable of potentially tripling the lifespan of regular consumers, it can be assumed that the practice of Bene Gesserit body balance could extend life and youth far beyond this. The Bene Gesserit have a commandment forbidding this practice, making violation of it punishable by death.
When the Lady Jessica returns to Arrakis from Caladan she is horrified to find that Alia has apparently not aged at all since the last time she saw her. She discusses this fact with Leto II, who confirms that her daughter has in fact accomplished the body balance necessary for her to live indefinitely. She fears that Leto might also engage in this practice, but he denies this.
Alia later describes to Duncan why this practice is referred to as the "Great Temptation" and why it was forbidden. If many Bene Gesserits extended their youth and life by balancing their internal chemistry, it would be noticed by the general population. When it was realized that Bene Gesserit adepts had the ability to live indefinitely without the aid of the spice, they would be kidnapped and tortured for their secrets: no member of the sisterhood would be safe. Alia states that the fact she has accomplished this body balance is the reason the Sisterhood calls her an Abomination.