It is best to ensure that all form inputs have a visible label. Visible labels help users understand the input's purpose. Placeholder text does not suffice and should not be considered as "good enough".
There are situations where a label for a form element can be hidden visually, given that there is still sufficient context about what the user should expect. For example, as search text input with a corresponding "Search" button, where "Search" is visible text in the button.