Coffee-ground vomiting is a clinical sign that indicates upper gastrointestinal (GI) bleeding.
- The vomitus appears dark brown or black, resembling coffee grounds, due to the presence of partially digested blood which has been altered by gastric acid.

Among the options provided:
1. Gastroesophageal reflux disease (GERD) primarily causes heartburn and regurgitation. It typically does not lead to significant bleeding that would cause coffee-ground vomiting.

2. Peptic ulcer disease (PUD) is one of the most common causes of upper GI bleeding. Ulcers in the stomach or duodenum can erode into blood vessels, causing bleeding. When the blood is partially digested by gastric acid before vomiting, it appears as coffee-ground material. Therefore, coffee-ground vomiting is most commonly observed in peptic ulcer disease.

3. Acute pancreatitis manifests mainly with severe abdominal pain and systemic inflammation. Vomiting in pancreatitis is generally due to ileus or gastric outlet obstruction, and it does not typically present with coffee-ground vomitus unless there is a concurrent GI bleed.

4. Mallory-Weiss tear refers to mucosal lacerations at the gastroesophageal junction, often caused by severe retching or vomiting. It usually causes fresh (bright red) hematemesis rather than coffee-ground vomitus because the blood is usually from a recent bleed and has not been digested.

Key points:
- Coffee-ground vomitus indicates upper GI bleeding with partially digested blood.
- Peptic ulcer disease frequently causes such bleeding due to mucosal ulceration and erosion of blood vessels.
- GERD and acute pancreatitis rarely cause upper GI bleeding.
- Mallory-Weiss tears usually cause fresh bleeding, not coffee-ground vomiting.