from man crontab:
Step values can be used in conjunction with ranges. Following a range with / specifies skips of the number's value through the range. For example 0-23/2 can be used in the hours field to specify command execution every other hour (the alternative in the V7 standard is 0,2,4,6,8,10,12,14,16,18,20,22). Steps are also permitted after an asterisk, so if you want to say every two hours, just use */2
As an example, I see:
~# cat /etc/cron.d/leafnode
# Cron entries for Leafnode.
# Run /etc/news/leafnode/do-fetch-news every hour.
8 * * * * news if [ -x /etc/news/leafnode/do-fetch-news ]; then /etc/news/leafnode/do-fetch-news; fi
now, every, let's say, two hours I want to run "killall fetchnews" because it has a tendency to stall. Would that look like:
#cron to kill defunct fetchnews
* */2 * * * killall fetchnews
(leaving out the logic of looking for a zombie process (if that's correct terminology))