It is the plural of Latin origin used mainly in formal contexts. In ordinary speech the -es suffix is commonly used:

[Catci vs cactuses][1]:

• Cacti is the Latin plural of cactus, and some writers use it in English. Cactuses is the English plural. Dictionaries list both, and neither is right or wrong. Also, like many names of plants, the uninflected cactus is sometimes treated as plural.

 
• The prevalence of the Latin cacti can be attributed to the influence of Latin on biological nomenclature.

 
• These Latin plurals are not considered out of place in botany and other scientific fields, and some make their way into broader usage, but there’s no good reason that the ordinary English speaker should have to abide by the rules of Latin grammar.

 
• Cactus is not the only Latin-derived English word ending in –us, and most are conventionally pluralized in the English manner. Fungus, like cactus, often becomes fungi (though funguses is just as good), but this is one of the few exceptions.

 
• Most English speakers don’t say ani instead of anuses, apparati instead of apparatuses, campi instead of campuses, octopi instead of octopuses, stati instead of statuses, or viri instead viruses, and there’s no reason cactus should be any didfferent. It’s a matter of preference, though, and cacti is not wrong.

(The Grammarist) [1]: http://grammarist.com/usage/cacti-cactuses/

It is the plural of Latin origin used mainly in formal contexts. In ordinary speech the -es suffix is commonly used:

Catci vs cactuses:

• Cacti is the Latin plural of cactus, and some writers use it in English. Cactuses is the English plural. Dictionaries list both, and neither is right or wrong. Also, like many names of plants, the uninflected cactus is sometimes treated as plural.

• The prevalence of the Latin cacti can be attributed to the influence of Latin on biological nomenclature.

• These Latin plurals are not considered out of place in botany and other scientific fields, and some make their way into broader usage, but there’s no good reason that the ordinary English speaker should have to abide by the rules of Latin grammar.

• Cactus is not the only Latin-derived English word ending in –us, and most are conventionally pluralized in the English manner. Fungus, like cactus, often becomes fungi (though funguses is just as good), but this is one of the few exceptions.

• Most English speakers don’t say ani instead of anuses, apparati instead of apparatuses, campi instead of campuses, octopi instead of octopuses, stati instead of statuses, or viri instead viruses, and there’s no reason cactus should be any didfferent. It’s a matter of preference, though, and cacti is not wrong.

(The Grammarist)