1Cooperation or collaboration: working together by two or more people or organizations (e.g., joint research, joint work).
二つの会社が共同で新製品を開発することになった。
The two companies decided to jointly develop a new product.
共同研究の成果は国際会議で発表された。
The results of the joint research were presented at an international conference.
被災地の復興は地域とボランティアの共同作業だ。
Rebuilding the disaster area is a joint effort of the community and volunteers.
2Communal use or common possession: shared ownership or use by multiple parties (e.g., communal housing, joint ownership).
このマンションは共同住宅で、共有スペースが多い。
This apartment is communal housing with many shared spaces.
土地は兄弟で共同所有している。
The land is co-owned by the brothers.
図書館の設備は地域住民の共同利用に提供されている。
The library's facilities are made available for communal use by local residents.