A character who is much smaller compared to average human size, typically less than a foot (30 cm) tall and often smaller.
Often accompanied by macro (if so, also tag size_difference). Note that neither character has to really be macro to e.g. get the micro_on_macro tag (and hence micro) since it is often impossible to determine which character, if any, is human-sized and which is larger/smaller.
Often seen together with vore.