An oath can be defined as:

An oath is fundamentally a solemn spoken pledge, where an individual makes a serious commitment, often invoking a higher power, to tell the truth or carry out an obligation. This definition is central to understanding what an oath entails in the context of notarial acts. In California, oaths are commonly used during various notarial transactions, particularly those involving affirmations of honesty and integrity, making the use of oaths not only a common practice but a necessary component in specific situations.

While affirmations—another form of commitment—serve a similar purpose and can often be used in lieu of oaths, they do not qualify as oaths themselves because they do not typically invoke a higher authority. This distinction is essential in understanding the broader context of notary duties, as oaths and affirmations serve different roles.

In conclusion, the notion that oaths are a solemn spoken pledge is the critical characteristic that defines them, and their application during notarial transactions further solidifies their importance in the notary public's responsibilities. Thus, both aspects combined support the answer.