Adware is code that forces a browser to display adverts the web designer did not intend to include in their page. So in this context you could define adware as 'malware' i.e. unwanted code within the browser.
Sometimes it is also called advertisment-supported software.
For example it may force intrusive pop-up adverts to be displayed. If the user clicks on the advert, this generates revenue for ad developer.
Adware code is present on both desktop computers and mobile devices.