Gittin, Chapter 7, Mishnah 8. The Mishnah deals with a get given on condition, and with the question of its status when the husband dies within the period he set in his condition.
The first case - "Harei zeh gitech im lo basi mikan v'ad sheneim asar chodesh":
One who gives a get to his wife and says to her: "Harei zeh gitech im lo basi mikan v'ad sheneim asar chodesh" - this is your get if I do not return within twelve months - "u'meis b'toch sheneim asar chodesh - eino get" - and he died within the twelve months, it is not a valid get. According to the way he phrased it, the get takes effect only once it becomes clear that he did not return, and therefore it can take effect only at the end of the twelve months. In this case the husband is no longer alive at the end of that period, and a get cannot take effect after death.
The second case - "Harei zeh gitech me'achshav":
But if he said: "Harei zeh gitech me'achshav im lo basi ad sof sheneim asar chodesh" - this is your get from now, if I do not return by the end of twelve months, meaning that the get takes effect retroactively from the moment it was given, on condition that he does not return within twelve months - "u'meis b'toch sheneim asar chodesh - harei zeh get" - and he died within the twelve months, it is a valid get. Even though he died within the twelve months, the get takes effect at the moment it was given, while the husband was still alive, and therefore it is valid.
The Gemara's question: must she wait twelve months?
The Gemara raises an interesting question here: must the woman wait until the twelve months have passed before she remarries, even though the husband has already died? The condition was that the get would take effect if he did not return within twelve months; since he died, he will certainly not be returning, and the get takes effect retroactively. Must she nevertheless wait out the entire period?
Tosafos explain the Gemara's question this way: do we make a decree in the case where the husband died, on account of the case where he did not die? Do we issue such a decree or not?
In summary: in this Mishnah we learned the difference between a condition phrased as "im lo basi" - where the get takes effect only at the end of the twelve months, so that if the husband dies within the period it is not a get - and a condition phrased with "me'achshav" - where the get takes effect retroactively from the moment it was given, and therefore it is a valid get. We also considered the Gemara's question of whether the woman must wait twelve months, and the explanation of Tosafos that the question is one of a decree lest the husband have not died.