If you want your content to be optimally findable in search engine results, you will not only have to follow certain guidelines to make the content highly searchable, but also make sure that the terms your target audience uses to search are reflected in your content. This section explains how to figure out these words.
Don't focus too much on keywords
But first, a side note: in the past, the challenge was to find exactly the right words to include in your content. The better the word in your text matched the search query, the better you scored in the search results. Nowadays, search engines, especially Google, are no longer focused on exactly matching a search query with the content of a page. It is increasingly about matching the intent of the person entering the search query, or in other words answering the underlying search question. Google is getting better at matching a search query with other terms, such as synonyms, the plural or singular form or the correct spelling when the word contains an error.
Nowadays, offering useful content is therefore very important, and finding 'the perfect term' of secondary importance. There is no such thing anyway, because even if you start from the word that is most searched for, you will miss visits from those who word it differently.
Furthermore, search engines, and therefore we too, increasingly use so-called 'structured data': an attribute given to a piece of content that tells search engines exactly what that content is and thus provides context. For example, the date of an event, the name of an author, etc. Where possible, these attributes are automatically provided by Drupal when you fill in the fields in the content types correctly.
So don't think of selecting search terms as a search for the optimal word that you can use on all your pages and thus score optimally in search results. Think of it as a search for words that match your target audience's needs and wording, and that indicate that your page answers their questions.
Moreover, search term optimisation is not an exact science. Try it out, monitor the results, and adjust where necessary.