1. semihourly - Adjective
2. semihourly - Adverb
(properly) Half-hourly: occurring every thirty minutes.
The warship rang its semihourly bell, announcing it was now half past 10 o'clock.
(inexact) Roughly (but not exactly) hourly: occurring around every sixty minutes.
(properly) Half-hourly: at intervals of 30 minutes.
Samples were collected semihourly (on the hour and half hour) throughout the study.
(inexact) Roughly (but not exactly) hourly: at intervals of around 60 minutes.
Source: en.wiktionary.org