First, a definition. An entry, sometimes also called an entry array, is a single unit comprised of the main heading and everything that goes with it, which can include subheadings, locators, and cross-references. It is complete unto itself.

Yet though complete unto itself, a single entry is not an index. An index, rather, is formed by multiple entries—often hundreds or even thousands of entries—placed together. It is this unity out of many that allows an index to point readers towards so many different pieces of information in the text.

To succeed in its function, an entry needs to be clearly written. The user of the index should understand exactly what they can expect to find if they go to where the entry points. Subheadings and cross-references are tools that can be used to achieve this clarity. 

The entry also needs to contain the sum total of everything the reader needs to know about that specific subject. There is no ideal length for an entry, so long as the entry contains everything it needs. Each entry will be unique. 

Having said that, since an index consists of entries in aggregate, there should be consistency in how entries are written and formatted. Subheadings should be written in a similar style and used for similar reasons, for example. Exceptions can exist, so that each entry can be the best that it can be, but overall there should be a sense of unity to the index, which stems from the entries working together.

It is also important to recognize that not all entries are equal. Metatopic and supermain heading entries, for example, will likely be expansive, reflecting the role of those topics in the text. Regular main headings and other lesser details will likely have simpler entries. If index length is an issue, some entries may have to be prioritized over others, by way of elimination or brevity.

An entry is quite elemental and basic to the index; it hardly seems worth talking about. Yet it is also the building block out of which the index is constructed. It is possible to write a poorly written entry, which obscures and confuses rather than illumines, which in turn will lead to a mediocre index. So I think it is worth asking, what makes a good entry? Get the basics right, and the rest of the index will follow.