请为我介绍十个新的单词以及它们的含义。
1. Pandemic- (noun) An outbreak of a disease that occurs over a wide geographic area and affects an exceptionally high proportion of the population.

2. Vexillology- (noun) The study of flags.

3. Epistemology- (noun) The study of knowledge and belief.

4. Ameliorate- (verb) To make something better or more tolerable.

5. Euphoria- (noun) A feeling of great happiness or well-being.

6. Hypothetical- (adjective) Of, based on, or serving as a hypothesis.

7. Melancholy- (noun) A feeling of pensive sadness, typically with no obvious cause.

8. Rejuvenate- (verb) Make someone or something feel or look younger or fresher.

9. Xenophobia- (noun) Fear or hatred of foreigners, people from different cultures, or strangers.

10. Abstinence- (noun) The practice of restraining oneself from indulging in something, often alcohol or sexual activity.