Matriliny is the term used when descent is traced through the mother. In matrilineal societies, kinship and inheritance are determined through the female line, with individuals belonging to their mother's lineage. This is in contrast to patrilineal societies, where descent is traced through the father, and individuals belong to their father's lineage.

Explanation:
1. Matriliny:
- Matriliny is a social system where descent, inheritance, and kinship are traced through the female line.
- In matrilineal societies, individuals belong to their mother's lineage, and children are considered to be part of their mother's family.
- Matrilineal descent is often associated with matrilocality, where a married couple resides with or near the wife's family.
- Matriliny is practiced in various cultures around the world, including some Indigenous societies in Africa, Southeast Asia, and the Pacific Islands.

2. Patriliny:
- Patriliny is the opposite of matriliny, where descent and kinship are traced through the male line.
- In patrilineal societies, individuals belong to their father's lineage, and children are considered to be part of their father's family.
- Patrilineal descent is often associated with patrilocality, where a married couple resides with or near the husband's family.
- Patriliny is more commonly practiced worldwide, including in many Western societies.

3. Exogamy:
- Exogamy refers to the practice of marrying outside of a specific social group or community.
- This can be based on various factors such as kinship, religion, or ethnicity.
- Exogamy helps to establish and maintain social connections between different groups.

4. Endogamy:
- Endogamy, on the other hand, refers to the practice of marrying within a specific social group or community.
- This can be based on factors such as caste, religion, or ethnicity.
- Endogamy helps to preserve cultural and social boundaries within a community.

Conclusion:
In the given question, the term used when descent is traced through the mother is matriliny. Matriliny is a social system where kinship, inheritance, and descent are determined through the female line. This is in contrast to patriliny, where descent is traced through the father.