Traditionally, the planning of operating reserve has been done in terms of capacity and linear (average) ramping requirements. On the other hand, the newly emerging concept of power system flexibility puts emphasis on resources' maneuverability. This is motivated by the need to offset the variability and uncertainty of increasingly more significant renewable power generation. Here, we re-think the fundamental meaning of reserve with respect to the concept of flexibility and present a new flexibility assessment framework. In doing so, we generalize the concept of flexibility envelopes with the objective of matching higher-order features of renewable generation variability and uncertainty and those of flexibility resources. We assert that flexibility adequacy is directly related to how well the aggregate flexibility envelope formed by flexibility resources encloses the flexibility requirement envelope and its dynamics over an operational horizon. We illustrate this concept with a simple flexibility-constrained unit commitment example.