المعنى الاصطلاحي :

Agreement of two things or more and their sharing the same description, place, type, or etc.

الشرح المختصر :

"Ittihād" (combination/union) means that two things or more get together so that they become a single entity. There are several types of "ittihād", including the following: 1. "Ittihād" of liability: a person becomes a creditor and a debtor at the same time. 2. "Ittihād" of a session: when there are two statements in the same session, like that of buying and selling. 3. "Ittihād" of kind: when two are of the same type or species, like a goat and a sheep; and both are considered "ghanam" (sheep; includes goats also). 4. "Ittihād" of measure/weight: two things may either be measurable or weighable. 5. "Ittihād" of ruling: when two things fall under the same ruling. For example, the excretion of urine and feces both prevent the person from offering the prayer. 6. "Ittihād" of reason: when two acts of robbery are committed by the same person and both require amputation of his hand.

التعريف اللغوي المختصر :

"Ittihād": joining and merging of two things and more. Opposite: "tafarruq" (disunity), "ta‘addud" (multiplicity). It is derived from "tawheed", which means making something a single unit. Other meanings: gathering, conjunction, agreement, coupling, intermixing, blending, integration, intersection, and connection.