To define Agreement? Many drafters define the term this Agreement (or, as appropriate, this Deed, this Amendment, etc.) in the introductory clause. Defining the term is, as such, not necessary: the mere article this in this agreement (used anywhere in the document) obviates the need for a defined term, because what other agreement would be referred to?
This is even more obvious if in the body text, an entire agreement provision is included stating that the Schedules and Annexes form an integral part of this Agreement and references to this Agreement shall include its Schedules and Annexes. Theoretically, the term this agreement could be interpreted to refer to that particular sentence or contract clause (e.g. the arbitration ‘agreement’) but (i) whether there is any relevancy in it at all, plus (ii) the likelihood that a party argues that this agreement refers to a part only, plus (iii) the likelihood of a court accepting this interpretation, is very remote. Despite it being redundant, personally I prefer to use Agreement as a defined term.
Do not list in article 1. If you do define (and capitalise) “Agreement”, do so in the title line on the first contract page or in the recitals. Do not define (or repeat or summarise its defined scope) in the definitions article. If anything needs to be elaborated on, do so in an interpretation section that also explains other references in the Agreement, or in an entire agreement clause in the miscellaneous article at the end of the contract.