Explanation:

- Philately: This term refers to the collection and study of postage stamps. It is the correct one-word substitute for the given group of words.

- Ornithology: This is the scientific study of birds, not related to postage stamps.

- Philosophy: This involves the study of fundamental questions such as those about existence, knowledge, and ethics, unrelated to postage stamps.

- Philanthropy: This is the act of promoting the welfare of others, often through generous donations of money, not related to stamps.