Final answer:
A fatty acid is best described as amphipathic because it contains both hydrophilic and hydrophobic regions, allowing it to interact with both water and nonpolar substances.
Step-by-step explanation:
The term that best describes a fatty acid is amphipathic. This is because fatty acids have both a hydrophilic region, which is their carboxylic acid group (-COOH), and a hydrophobic region, which is the long alkyl chain. The hydrophilic part is attracted to water, while the hydrophobic part repels water, making them insoluble in water. Phospholipids, which contain fatty acids, are also described as amphipathic molecules because they have a hydrophilic phosphate group (the polar head) and two hydrophobic fatty acid chains (the nonpolar tails).