If you use (Manage display) format: hidden (not label: hidden), the field will not show for this display for this content type. The field could still show in another display (like for teasers). So you will need to set the field format to hidden for all displays.

It is still possible to make a view with the Views module that displays the field. It depends on how you set up the view. If you set up a view of teasers, the standard teaser display is used (and the field is hidden according to the display settings). If you make a view of fields, you just have to avoid picking the "secret field".

If this is an existing Drupal site where this content type and field is a part of many different views and/or other modules, you will need to test this out. Also if the field is searchable or hardcoded in some template.

If you use, on Manage display the hidden format (not label: hidden), the field won't show for the display mode for that content type. The field could still be shown in another display mode, for example the one for teasers. You need to set the field to hidden for all the display modes.

It is still possible to make a view with the Views module that displays the field. It depends on how you set up the view. If you set up a view of teasers, the standard teaser display is used (and the field is hidden according to the display settings). If you make a view of fields, you just have to avoid picking the "secret field."

If this is an existing Drupal site where this content type and field is a part of many different views and/or other modules, you will need to test this out. Also if the field is searchable or hard-coded in some template.