PubMed

Once you have entered search terms, you can sort them 3 different ways:

• Best Match (default option) - This is "based on an algorithm that analyzes each PubMed citation that includes the search terms. For each search query, "weight" is calculated for citations depending on how many search terms are found and in which fields they are found. In addition, recently-published articles are given a somewhat higher weight for sorting. The top articles returned by the weighted term frequency algorithm above are then re-ranked for better relevance by a new machine-learning algorithm. If any technical constraints prevent the machine-learning architecture from re-ranking the results, the sort will revert to the standard best match algorithm."
• Most Recent - This is the date a record was initially added to PubMed, not the publication date.
• Publication Date - The date that an article was published.

To change how your results are sorted, click on Display Options and choose from the "Sort by" dropdown menu.

Additionally, you can also change the number of items displayed per page and showing the entire abstract (as opposed to just the record summary).

Source: https://www.ncbi.nlm.nih.gov/books/NBK3827/