The blockchain height is the number of blocks that have been mined in the blockchain. For a fork to happen, the blockchain height must be at least 51%.
Other related questions:
Q: What is the height of a blockchain?
A: The height of a blockchain is the length of the chain, measured in blocks.
Q: What happens when a blockchain forks?
A: A blockchain fork occurs when a cryptocurrency’s existing code is changed, creating two separate versions of the blockchain with different rules. Forks can be temporary or permanent. Temporary forks happen when two miners find blocks at nearly the same time and the blockchain splits into two branches. These forks are often resolved quickly as the chain with the most work (the longest chain) is usually accepted as the valid one by the network. Permanent forks happen when the code is changed deliberately, creating two different versions of the cryptocurrency that can never be reconciled.
Q: What of participants have to agree for a soft fork to take place?
A: A soft fork is a change to the Bitcoin protocol that is backwards-compatible with older versions of the software. This means that users running the old software will still be able to participate in the Bitcoin network and validate transactions, but will not be able to mine blocks or earn rewards. In order for a soft fork to take place, a majority of miners must signal their support for the change.