The terms "mobile data" and "Wi-Fi" both have to do with distributing different types of digital data throughout wireless networks. However, the definitions of these terms are fundamentally different.
Mobile data consists of many different kinds of data that are sent and received over wireless platforms. Sometimes, mobile data can be called wireless data, and it is often defined as data that includes all sorts of data transmission that supports different kinds of information and interfaces. However, that definition of mobile data excludes voice transmissions, which are provided in fundamentally different ways.
In other words, data is not voice, and voice is not data. This definition of mobile data is supported by the ways that telecom companies bill consumers for data and voice plans. In fact, it's this separate billing structure that's likely to lead to a more collective understanding that data and voice are generally two separate service components.
Wi-Fi is a little different. Technical definitions of Wi-Fi might refer to it as a system supporting the ways that computers interact through wireless networks. To put it simply, where mobile data is the commodity being transferred, Wi-Fi tends to describe the service and set of interfaces that cooperate with one another to make that transfer possible. For example, suppose that a piece of mobile data is generated by a user typing on a device keyboard or touch screen. A Wi-Fi connection sends that individual piece of mobile data from one device to another through a complicated network of cell towers, IP connected stations and other types of network architecture.
Companies and professionals apply all sorts of security and data handling applications to Wi-Fi systems that deliver mobile data in various ways. The analysis of mobile data and the observation of Wi-Fi systems are becoming a bigger part of our world as wireless device contracts proliferate and the data telecom industry grows.