A wearable IoT device is a device that can be worn on the body and is connected to the internet to collect and transmit data. A fitness tracker is a prime example of a wearable IoT device, as it's worn on the wrist to monitor health and activity data like steps, heart rate, and sleep patterns.

Microsoft Azure IoT provides a fully managed service for connecting, processing, and storing data from IoT devices. While other cloud platforms offer IoT services, this is a distinctive feature of Azure IoT.

A zero-day vulnerability is a security flaw in a system or device that is not yet known to the vendor or the public. It's called "zero-day" because there are zero days of protection against it. Hackers can exploit such vulnerabilities before the vendor has a chance to fix them, making them high-priority issues for security.

A company's IoT devices participating in a DDoS attack could be due to a lack of proper security measures. IoT devices, if not adequately secured, can be vulnerable to malicious actors who compromise their operation and use them as bots in a DDoS attack. Ensuring security measures like encryption, strong authentication, and regular security updates is crucial in such cases.

Data breaches are a common security threat in IoT, as unauthorized access to IoT data can have serious consequences, such as privacy violations and misuse of sensitive information.

In the context of IoT, AI algorithms that enable devices to learn from data and enhance their performance over time are referred to as "Reinforcement Learning" algorithms. These algorithms learn by trial and error, which is valuable in IoT applications.

Wearable devices primarily use the "Bluetooth" communication model to send health metrics and data to the user's smartphone or other devices. Bluetooth is a short-range wireless communication technology commonly used in wearables for connectivity.

A temperature sensor would be the most appropriate choice for monitoring the shipment of perishable goods. Temperature sensors, often referred to as thermometers or thermocouples, measure the temperature of their surroundings. In this context, they are crucial for ensuring that perishable items remain within a safe temperature range during transportation, such as in refrigerated trucks or containers.

Data analytics in IoT is instrumental in gaining insights from the vast amount of data generated by IoT devices. It helps in detecting anomalies, predicting trends, and making informed decisions, which are essential for optimizing operations and achieving the goals of IoT applications.

By the year 2015, the number of connected devices had already surpassed the global human population. This milestone highlights the rapid growth of IoT, where everyday objects and devices are becoming smart and interconnected, contributing to the data-driven world we live in today.