Final answer:
The term that describes something 'lasting only a very short time; fleeting' is 'transient.' This term relates to things that are short term and subject to change, which fits the context of fleeting moments in literature or the transience of memory as seen in Nathan's mother's example.
Step-by-step explanation:
The student is asking for the term that describes something that lasts only a very short time or is fleeting. Among the options given (tapered, transient, tacit, tangible), the correct term is transient. This word is defined as something that is short term and easily changed, matching the definition of something that lasts for only a brief period. This contrasts with something that is long term, stable and not easily changed.
The concept of transience can be seen in literature, such as the passage provided where fleeting moments and subtle details are keenly observed and noted, perhaps because they are so ephemeral. It is also relevant in the context of memory, as illustrated by the example where Nathan's mother could not remember the details of 'To Kill a Mockingbird' due to the transience of memory.