I see the gnucash tag, so I guess you're setting up accounting software? Are you setting it up for you or for you and your wife as an "entity"? If it's the former, then I guess your wife's contribution is an equity contribution. If it's the latter, then her separate account should also be in the system, and it's a transfer.
In light of the comments, some additional thoughts:
- I think it's senselessly complicated to use this type of accounting for personal use, but you're definitely not the only person posting here that's doing it, so other people must disagree.
- I think that, given what you've said you want to do, "equity" makes more sense than anything else, but this is not a unique choice. If you think about this is a business analogy, your wife's contribution to this account is more like becoming a shareholder than anything else, because your model seems to be that she still owns half of the joint account.
- It would be more appropriate from an accounting point of view to either: (a) Include all of your wife's accounts and make the file be accounting at the level of the marriage, or (b) Make the file be just for yourself and remove the joint account as a regular bank account. If you do (b), then then you'd represent your share of the joint account as an asset, but you would not keep its full balance "on your books" since you don't own it all.
On the last bullet, option (b), this joint account would be like if you had a business that owned shares in some other company. You'd only take credit for your shares in the other company, not its full value.