Final answer:
The split row field in the given Splunk pivot search command is 'failed_request', as it is the field used to organize the counts of failed activities in the pivot report.
Step-by-step explanation:
In the context of the provided pivot search command from Splunk, which is used for creating reports and dashboards from your data, the split row field defines how the results should be separated or organized in rows based on a particular field from the dataset. In the given search example, the command "| pivot Buttercup_Games_Site_Activity failed_request count(failed_request) AS "Count of Failed requests"" is breaking down the site activity data by the 'failed_request' field. This field is the split row field, as the pivot is likely organizing the counts of failed requests separated by different criteria or values within the 'failed_request' field.