A connecting vowel.
In Latin this is a regular part of word formation. As a result, it occurs in many words borrowed into English either directly from Latin or through French, for example omnivorous, uniform, and pacific. On the model of these and others, it has come to be used in words created in English. These can be formed from words which are ultimately of Latin origin, such as calciferous (containing or producing calcium salts, from Latin calx, calc-, lime), ultimately from Greek, such as amoebiform (like an amoeba, from Greek amoibē, change, alternation), or even from those not of classical origin, such as tickicide (a substance that kills ticks, see -cide). It occurs particularly before an ending of Latin origin such as -ana, -ferous, -fic, -form, -fy, -gerous, or -vorous (see -vore). See also -o-.