Why does the Declaration of Independence use the word “unalienable” to describe the rights listed in the preamble?

"Unalienable rights" or "inalienable rights" is another way to describe "natural rights." This is a reflection of the Founding Fathers' ideology. They believed that all human beings are given a certain set of rights by God. They are natural in that they belong to us not by virtue of our being American, French, British, or any other nationality, but simply because we are human. Because these rights are God-given and natural, they cannot legitimately be interfered with by any human institution, such as government. In other words, they are "inalienable."
Jefferson included "life, liberty, and the pursuit of happiness" as among our inalienable rights. If we examine each one of them carefully, we can see that they are all necessary components of what most people would regard as essential to our humanity. We cannot truly flourish as human beings if our right to life is in any way violated. Nor can we can do so if we are restricted from using our innate liberty to pursue our own projects and goals in life, "the pursuit of happiness" in the Declaration's famous words.
The biggest threat to our inalienable rights is the unchecked power of government. Government, unlike our inalienable rights, is artificial. It exists to serve and protect our natural rights. The Founding Fathers set about constructing political institutions that would fulfill this purpose. This warranted the establishment of a limited government with a system of checks and balances in order to prevent any one branch of government from getting too powerful and potentially abusing its power to threaten our inalienable rights.