Both pan- and omni- mean all, but pan- is Greek and omni- is Latin.
Wiktionary’s definition of pansexual and omnisexual suggests that when people distinguish them, it is that pansexual implies everyone in a gender-blind way, and omnisexual implies everyone but differently for different genders.
pansexual, adjective: someone who is attracted to all types of people regardless of gender or sex
omnisexual, adjective: being attracted to all genders (sometimes distinguished from pansexual by saying omnisexual attraction may still take account of gender or prefer particular genders)
If we were going strictly by prefixes, then it would be:
bisexual - attracted two precisely two genders,
multisexual - attracted to two or more genders,
omnisexual - attracted to all genders (though possibly to varying degrees),
pansexual - mixes Greek and Latin and therefore doesn’t count as a real word. (/joke)
Both pan- and omni- mean all, but pan- is Greek and omni- is Latin.
Wiktionary’s definition of pansexual and omnisexual suggests that when people distinguish them, it is that pansexual implies everyone in a gender-blind way, and omnisexual implies everyone but differently for different genders.
If we were going strictly by prefixes, then it would be: