What does it mean to be in "Flow"?

1) Completely involved, acutely-focused, highly-concentrated - with a disappearance (or at least quieting down) of the internal peripheral incessant 'chitter-chatter'
2) Sense of ecstasy - of being outside everyday reality
3) Great inner clarity - knowing what needs to be done. Recognizing/realizing how well everything is in fact going, i.e. having gratitude for all of one's blessings vs. over-emphasizing the perceived lackings.
4) Complete compatibility between task and skill/knowledge level. Neither anxious nor bored.
5) Sense of serenity - no worries about self, feeling of growing beyond the boundaries of ego - afterwards feeling of transcending ego in ways not thought possible
6) Timelessness - thoroughly focused on present, don't notice time passing
7) Intrinsic motivation - whatever produces "flow" becomes its own reward

Desirable side effects: disappearance of self-consciousness and insecurities
Undesirable side effects: once you know this state, it makes being ok with the steady state of being scattered, distracted, and anxious that much harder.

The 'approach' to Ao Nang Tower, a free-standing limestone pillar off the coast of Ao Nang Thailand (SW Coast/Andamen Sea). Gaining the route involved standing on the boat's roof and pulling up onto the rock (high tide only). Descent via rappel into either sea or boat.

Notice shadow cast by pillar and kayaker below. This was our friend Trampas who was taking photos to send us later (never did get those). I'm bottom, middle of photo.

Part-way up. 

Nearing the top.

Same shot, different angle. 

Cathedral Peak, Tuolumne Meadows, Yosemite NP. 

Atop Cathedral Peak, Tuolumne Meadows, Yosemite NP. 

Brad on Middle Cathedral, Yosemite NP. El Capitan in background. 

last pitch of Royal Arches, Yosemite NP. Half-dome in background. 

Low down on El Capitan, Yosemite NP. Brad leading. 

Approaching Hallett Peak, Rocky Mtn National Park