What exactly constitutes a Holy Ground in Highlander canon?

I have no concrete, specific, reference for this, but every place referred to in either the movies or TV series as holy ground has had some kind of marker visible, whether it be a grave marker, a carved stone, or active religious ceremonies being performed on the premises, so I think it would be a fair assumption that, in order to be holy ground, a site must be marked as such and/or be a location where organized religious ceremonies take place.

If you count the audio books as canon, there is STILL no definition given.. but they do indicate that the immortals have a natural ability to 'sense' being on holy ground, much like the sense that tells them that another immortal is near.

From the Highlander Wiki:

That said, in Kurgan Rising Duncan is lured into taking another immortal's head on consecrated ground. Macleod notes that he should have instinctively realized the ground was consecrated implying that the immortals have an ability similar to their Buzz that acts as a warning system.

"Duncan: But the thing that I don't understand, is why I didn't realise Justino had consecrated the building. Why didn't I sense that I was standing on holy ground? I should have known instinctively.

This may not be much help, but the franchise tends to play a bit fast and loose with what they consider "holy"; it can be a blessed, consecrated, sacred, significant or burial ground of any faith or people, it seems.

For example of this, in the Highlander:The Series season 3 episode "The Samurai", I don't believe the family shrine of Hideo Koto, was an established "burial ground", but because it had been revered and his body had been buried there, it was considered "holy". MaCleod and the immortal he fought, Micheal Kent, merely walked a few yards away, and were no longer "on" holy ground. It was never established whether they instinctively "knew" they were out of range, or if there was some boundary they had to pass.

For that matter, the only in-series recollection I have of immortals fighting on holy ground [and the resulting consequence] was Connor MaCleod fighting with Kane in a Buddhist Shrine in Highlander III: The Sorcerer , where the blades of the immortals were destroyed and they were whiplashed by wind and energy.

So again, it could be as simple as "any ground for which people have reverence", as opposed to something specifically "made holy" through an act such as burial or consecration.

There is no "word of god"

From incidents in the movies and series it is shown that merely being a burial ground/grave does not make the ground holy. Methos, in a flashback, kills another nameles immortal while literally standing on the site of the mass grave of people who died in one of his earliest memories.

However, the following are shown as holy ground at various points in the movies and series:

• several christian churches
• a church graveyard
• a Buddhist shrine
• a family (Shinto) shrine
• a stone circle
• a watcher-owned building specially concecrated

From this it appears that holy ground is any area consecrated for religious purposes or used for regular religious prayer/worship and/or ritual, regardless of the religion.