"fun" can mean many things in many contexts. However, instead of trying to find a definition that suits all purposes (or at least does not exclude any valid perspective), we decided to leave it undefined.

When running experiments or testing computational systems, it may be best to substitute a term that can be measured objectively. Two examples were provided:

• Engagement. Like immersion, there may be metrics for measuring engagement.
• Replayability. As a metric, if someone is willing to play something more than once, there must be intrinsic value being derived.

Likely there are other measurable terms that can be substituted.

Subjective measures of "fun" can also be used, such as asking if someone is having fun, or how much fun they are having. It is important to note that people may use different definitions or different subjective measures.