Human Sacrifice

Human Sacrifice (人柱, Hitobashira) is a term in the Fullmetal Alchemist franchise used by the Homunculi to describe a certain, significant type of person. Due to the widely divergent paths taken by the manga storyline and the 2003 anime storyline after the term was introduced, it means two very different things in either continuity. 

In the manga/2009 anime
A Human Sacrifice is defined as an alchemist who has passed through the Gate by way of Human Transmutation and survived its after effects. This achievement requires a remarkable amount of skill in being capable of opening the Gate, courage in being willing to open and enter the Gate despite the hazards and luck in being able to exit the Gate without something vital being taken as toll and, as such, Human Sacrifices and candidates for Human Sacrifice are exceptionally rare. The Gate and the Truth, which resides within, appear to recognize those who have passed through it and call out to them when activated in their presence. It is this particular trait that makes these Human Sacrifices so important to the Homunculi and their Father, as their overarching plan requires that they possess five of these people on the Promised Day. While the plan, in its smoothest form, involves seeking out and protecting five Human Sacrifices until the Promised Day has come, it appears that a contingency exists within their plan to allow for the creation of a Human Sacrifice by forcing a candidate through the Gate at the risk of losing a Homunculus in the process. When the five Sacrifices are gathered and the Promised Day reaches its pinnacle of significance, each of the five individuals' Gates is opened, causing them to resonate and conflict with each other, releasing a massive amount of energy which will allow Father to open the Gate of the Earth itself. Contrary to what the name suggests, the Human Sacrifices don't actually die and survive with no ill effects. In fact, the Sacrifices were some of the few humans to not have their souls dragged from their bodies.

The Five Human Sacrifices are: Also, the Alchemist Jude technically qualifies as a human sacrifice due to his attempt to transmute Rosalie Harbinger. It is unknown why he was not selected to serve as an official sacrifice, although it could be his quiet life allowed him to drop below Father's radar. Julia Crichton qualifies as well due to her attempt to transmute life back into her brother, though she only succeeded due in part to his still being alive at the time. Mustang may have been responsible for keeping her below Father's radar as well and thus not selected to serve as an official sacrifice. In addition, Dr. Marcoh was a prime candidate for Human Sacrifice (alongside Roy Mustang who did become one). This was the reason they kidnapped him and kept him alive until his escape with Scar. It seemed he never committed the taboo like the Elric brothers or Izumi Curtis, nor in some way open the gate or see the Truth the way Hohenheim experienced. Instead, it is possible they intended for him to forcibly open the gate, either by killing someone and having him resurrect them or forcing him to as they did with Roy. This isn't further explored.
 * Van Hohenheim - Brought through the Gate as a result of the activation of the Nationwide Transmutation Circle around the nation of Xerxes. Gained immortality as a result, but his body also became the container of half the country's now dead population's souls.
 * Izumi Curtis - Opened the Gate through Human Transmutation in an attempt to resurrect her stillborn child. Some of her internal organs were removed as a result, and she was rendered infertile.
 * Edward Elric - Opened the Gate through Human Transmutation in an attempt to resurrect his late mother, losing his left leg as a result. Reopened the Gate to bring back the soul of his brother in exchange for his right arm.
 * Alphonse Elric - Brought through the Gate in the aftermath of his attempt to resurrect his late mother. Lost his entire body as a result, his soul bound to an empty suit of armor by his brother.
 * Roy Mustang - Forced through the Gate by the Homunculus Pride when the Promised Day found only four full Human Sacrifices in existence. Lost his eyesight as a result.

In the 2003 anime
A Human Sacrifice is defined as an alchemist skillful and desperate enough to create a Philosopher's Stone at the cost of many human lives. These people are significant to the Homunculi and their master Dante because the Stone will allow Dante to switch from her rotting body into a fresh new container and has the ability to make the Homunculi fully human.

Trivia

 * Pride noted that the human sacrifices were extremely foolish, as he pointed out the flaw in Alphonse's logic. Yes, they could have left the country and hid until the Promised Day passed; however, they made the decision to remain in Amestris and fight, making it easier for them to be captured and used.
 * The Homunculi only had a backup plan for a single sacrifice to be created; if the quota was lower than four at the time they did this, their efforts would be for nothing.