(The literature is thin on this one) The primary stakeholder (on-site, in-the-lab) of the system undergoing the XP methodology. Provides the final acceptance criteria for any behavior in the system. Sometimes a team instead of one person. Less optimally, but often likely, this is a Business Analyst who, while they may be a Domain Expert, does not have the final say, but acts as the advocate of the real Customer who is not on-site. (Beware the Customer who knows the Business but does not have the Final Say).
a quick (typically minutes or hours) exploration by coding of an area the development team lacks confidence in. The spike is concluded when you learn what you needed to learn. So-called because a spike is "end to end, but very thin", like driving a spike all the way through a log. See SpikeSolution, ArchitecturalSpike, SpikeDescribed.