The phrase "turnout threshold" refers to the minimum number of people required to participate in an event or an election for it to be considered valid or successful. Full definition
The bill includes a requirement for a 40 % turnout threshold for strike ballots in «important public services». (bbc.co.uk)
And in a bid to throw red meat to his core vote, Cameron is now floating the idea of applying turnout thresholds to trade union strike ballots. (labour-uncut.co.uk)
They include the introduction of a 50 % turnout threshold on strike votes, a change to allow employers to bring in agency staff to cover work during industrial action, and a requirement for strike leaders to wear armbands on the picket line. (politics.co.uk)