Aristotle - Categories - Ch. 13 - Meanings of the Word "Simultaneous"

There are three meanings of the word "simultaneous" which Aristotle discusses in Chapter 13. (1) Simultaneous in respect to time, (2) simultaneous as to the implication of existence, and (3) simultaneous as coordinate species of the same genus. 

Simultaneous In Respect to Time (Without Qualification)

This type of being simultaneous is without qualification and most strict as it refers to two things which happen at the same time, with neither being before or after the other. For example, twins being conceived. 

Simultaneous as to Implication of Existence

This refers to two things which may not be simultaneous in time, but they will exist together. Neither is the cause of the other's existence, but they come together, such that if one exists the other exists as well. For example, double and half. If there is half of something, there also will be double of that thing and vice versa. 

Simultaneous as Co-ordinate Species of the Same Genus

Things can also be called simultaneous when they split from the same level of genus. For example, the genus animal will split into bird, fish and beast. Genera are prior to species and so that's why we cannot say they are reciprocal in existence. If you have a fish, you have an animal. But if you have an animal you do not necessarily have a fish. 

Text of Chapter 13

14°24. Those things are called simultaneous without qualification and most strictly which come into being at the same time; for neither is prior or posterior. These are called simultaneous in respect of time. But those things are called simultaneous by nature which reciprocate as to implication of existence, provided that neither is in any way the cause of the other’s existence, e.g. the double and the half. These reciprocate, since if there is a double there is a half and if there is a half there is a double, but neither is the cause of the other’s existence. Also, co-ordinate species of the same genus are called simultaneous by nature. It is those resulting from the same division that are called co-ordinate, e.g. bird and beast and fish. For these are of the same genus and co-ordinate, since animal is divided into these—into bird and beast and fish—and none of them is prior or posterior; and things of this kind are thought to be simultaneous by nature. Each of these might itself be further divided into species (I mean beast and bird and fish); so there, too, those resulting from the same division of the same genus will be simultaneous by nature. Genera, however, are always prior to species since they do not reciprocate as to implication of existence; e.g. if there is a fish there is an animal, but if there is an animal there is not necessarily a fish. Thus we call simultaneous by nature those things which reciprocate as to implication of existence provided that neither is in any way the cause of the other’s existence; and also co-ordinate species of the same genus. And we call simultaneous without qualification things which come into being at the same time.