Happening hourly, but at 30 minutes past each hour; ie, 08:30, 09:30 etc. / Happening on the hour and at 30 minutes after the hour. / (soccer) At thirty minutes into a half.

A period of time equal to thirty minutes, half an hour. / The time of thirty minutes past any particular hour.

plural of half-hour

Once every half-hour, every 30 minutes.

At intervals of a half-hour, every 30 minutes.

which lasts half an hour, or approximately so.