Under Section 7 of the Arbitration and Conciliation Act, 1996, an arbitration agreement is a written agreement by which parties agree to submit present or future disputes to arbitration. It can be a clause within a larger contract, or a separate standalone agreement.
"In writing" is interpreted broadly — a signed document, an exchange of letters, telex or other means of communication, or an exchange of statements of claim and defence in which one party alleges and the other does not deny the existence of the agreement, can all satisfy the requirement.
A valid arbitration agreement need not be lengthy, but it must clearly show the parties' intention to arbitrate rather than merely to consider arbitration as an option — vague language ("parties may refer disputes to arbitration") has repeatedly been held insufficient to actually bind the parties to the process.