Which term signifies a surgical opening from the ileum?

The term that signifies a surgical opening from the ileum is "ileostomy." This procedure involves creating an opening in the abdominal wall through which the end of the ileum, the final segment of the small intestine, is brought to the surface of the skin. This allows for the diversion of waste material out of the body when the normal route through the colon is not possible due to disease, injury, or surgical removal of part of the large intestine.

In contrast, the other terms represent different surgical procedures. "Colostomy" refers to an opening created from the colon, "gastrectomy" is the surgical removal of part or all of the stomach, and "appendectomy" involves the surgical removal of the appendix. Thus, the specificity of the term "ileostomy" in relation to the ileum makes it the correct answer for this question.