51% Attacks (Risk View)
From a user’s perspective, a 51% attack risk is the chance that a small network’s recent transactions can be reversed by a majority miner or validator.
In depth
Large chains like Bitcoin and Ethereum are effectively immune because the cost is astronomical, but several smaller Proof of Work coins have been successfully attacked and double-spent against exchanges. Exchanges respond by requiring many more confirmations for such assets. It is a reason to be cautious with low-hash-rate coins.
Why it matters
It explains why deposit confirmation counts differ so much between assets.