Stands for "Over-The-Air." OTA refers to any type of wireless transmission, but it is most commonly used to describe either 1) software updates distributed to mobile devices or 2) TV and radio broadcasts transmitted over the air.
1) Mobile Device Programming
OTA signals that are used to program, update, configure, and provision hardware devices are collectively known as OTAP (Over-The-Air Programming). One example is over-the-air activation (OTAA), which allows you to wirelessly activate a new cell phone by simply typing a code or following specific steps. This automated service provided by most mobile providers can save you a trip to a mobile phone service location when you buy a new phone. OTA transmissions can also send updates such as new carrier settings and even OS updates to supported smartphone.
Another example is over-the-air service provisioning (OTASP), which is used to remotely activate and configure multiple devices, such as radio handsets, for a specific group of users. OTASP transmissions are typically sent out from a central server and may be encrypted to protect personal information. Both OTAA and OTASP are subsets of OTAP.
2) TV and Radio Signals
OTA can also refer to television and radio transmissions that are sent and received over the air. Most often, OTA is used to contrast television channels that are broadcast over the air with those offered by cable service providers. OTA channels are local channels that can be received using a traditional "bunny ears" antenna. These types of channels are free and do not require a monthly subscription. Most cable providers include digital versions of OTA channels that are specific to each customer's region.
NOTE: Even though satellite TV broadcasts are transmitted wirelessly, the channels offered by satellite providers are generally not considered "OTA." Instead, they are simply called "cable channels," since they are the same channels offered by cable companies.