Does phasing count as leaving or entering the battlefield for triggered abilities?

When a permanent phases out, it is treated as though it does not exist, but it never technically leaves the battlefield — and when it phases back in, it never technically enters the battlefield. This is the core rule of phasing, found in CR 702.45e: 'A permanent that phased out is treated as though it does not exist... It does not leave the battlefield.'

Because phasing out is not a zone change (the permanent stays in the battlefield zone the whole time), triggered abilities that watch for a permanent 'leaving the battlefield' — such as 'when this creature dies' or 'when ~ leaves the battlefield' — do not trigger when a permanent phases out. Similarly, abilities that trigger when a permanent 'enters the battlefield' do not trigger when a permanent phases back in (CR 702.45d).

This also means auras and equipment attached to a phasing permanent phase out with it automatically, and they return still attached — because no 'leaves the battlefield' event occurred to cause them to fall off (CR 702.45c).

Concrete example: You control a Grimdancer enchanted with Pacifism. An opponent phases out your Grimdancer with Teferi's Protection. When Grimdancer phases back in on your next untap step, Pacifism is still attached to it, and no 'enters the battlefield' triggers fire — not even your own abilities that say 'when Grimdancer enters the battlefield.'

In summary: phasing is deliberately designed to skip 'leaves/enters the battlefield' triggers entirely, making it a unique form of temporary removal that sidesteps many common game interactions (CR 702.45d, 702.45e).