Yes, it is, as long your site is accessible to your end users without charge. For example, if your website is supported by advertising, it likely falls within the terms of the AJAX Search API.
However, not all commercial uses are allowed. For example:
- Premium websites – If your site is only available to paying customers, you cannot use the AJAX Search API.
Remember, Google reserves the right to suspend or terminate the use of the service at any time, so please read the terms of use carefully.