New Button in Look-up Window

When you create a new Look-Up or Master-Detail field on any custom object or on any standard object and then during look-up you see a New button clicking on it will allow you to create a new record on run from look-up window.

Before discussing more about it few important points to note :

1. This button is available only for Account and Contact.

New Button for Account Look-up

New Button for Contact Look-up

No New Button for Opportunity, same for other custom and standard objects

New Button for Master-Detail relationship with Account, same for Contact

2. Record created by clicking New button on look-up window will bypass validation rules.

If you want to remove this "New" button from lookup window then you can do it from Setup -> Customize -> User Interface and deselect "Show Quick Create " and save the settings.

after deselecting check the lookup window to confirm if New button is removed.

NOTE : Deselecting the check-box will disable Quick creation of Account, Contact and Opportunity from side panel.