What it was
GHash.IO was a bitcoin mining pool. Its own press office says it "entered the mining market in July 2013" and that it shared account credentials with the exchange CEX.IO, so that "the pool is considered to be the mining side of CEX.IO". A user could point hardware at it or buy hash rate on the exchange and have it mined at the pool. There was no pool fee. In July 2014 the front page advertised 43.15 petahashes per second.
What the numbers actually showed
Pool share is inferred from the marker a pool leaves in the blocks it finds, and in 2014 the figure everyone read was blockchain.info's hashrate distribution page. The copy archived on 13 June 2014, covering the previous 24 hours, puts GHash.IO at 49 per cent: 77 blocks against 46 for the six other identified pools combined. Reporting at the time said the pool had crossed half the network. No archived copy of that page shows a figure above 50 per cent and the pool never published one, so 49 per cent over a single day is what this entry states.
A day is not a state. The same page's default window was four days, and the copy archived on 17 June 2014 shows 32 per cent. Other copies of that four-day view give 33 per cent on 22 January 2014, 36 per cent on 31 May 2014 and 34 per cent on 13 July 2014. The pool sat in the thirties and spiked.
What the pool said
On 16 June 2014 GHash.IO issued a statement: "We never have and never will participate in any 51% attack or double spend against Bitcoin." It declined to cap itself, arguing that "successful and innovative companies cannot be expected to limit their growth or competitiveness as a direct result of their success", and that charging a fee to push miners away would only move the problem to whichever pool grew next. It proposed a round table with the leading pools and the Bitcoin Foundation instead.
That meeting happened in London on 9 July 2014. A week later the pool reversed itself and committed to "not exceed more than 39.99% of the overall Bitcoin hashrate", a figure a spokesperson confirmed to TechCrunch had been proposed at the round table and accepted by management.
It shrank after that. Its advertised hash rate was 2.36 petahashes per second by 28 August 2016, and by 4 November 2016 the site had been replaced by a notice in the past tense pointing visitors to CEX.IO.