Defining what constitutes "life" is a complex and multifaceted question that has puzzled philosophers and scientists for centuries. There is no single, universally accepted definition, and the criteria for life are constantly evolving as we gain new knowledge about the natural world. However, some common characteristics that are generally considered essential for something to be considered alive include:

1. Organization: Living organisms are highly organized, with specialized structures and systems that work together to maintain life. This includes cells as the basic unit of life, tissues, organs, and organ systems.

2. Metabolism: Living things carry out a wide range of chemical reactions that sustain life. This includes processes like respiration, photosynthesis, and nutrient breakdown.

3. Growth and Development: Living organisms grow and develop over time, changing in size, shape, and complexity. This involves processes like cell division, differentiation, and maturation.

4. Responsiveness to Stimuli: Living organisms can sense and respond to changes in their environment. This includes responding to light, temperature, chemical cues, and physical contact.

5. Reproduction: Living organisms are capable of producing offspring, ensuring the continuity of their species. This can involve sexual or asexual reproduction.

6. Adaptation: Living organisms can evolve over time, adapting to changes in their environment. This includes changes in genetic makeup that improve survival and reproductive success.

7. Homeostasis: Living organisms maintain a stable internal environment, despite fluctuations in the external environment. This involves regulating factors like temperature, pH, and water balance.

Beyond these basic characteristics:

* Information processing: Many living organisms process information from their environment and make decisions based on that information. This includes complex nervous systems in animals and simpler signaling mechanisms in plants.

* Emergent properties: Living organisms exhibit properties that are not present in their individual components. For example, consciousness in humans emerges from the interaction of billions of neurons.

It's important to note that not all living organisms exhibit all these characteristics to the same extent. For example, viruses are a gray area, as they can replicate and evolve but lack some other key characteristics of life.

Ultimately, the question of what makes something alive is open to debate. As our understanding of biology expands, the definition of life may continue to evolve.