A reasonable question to ask; and one that should more often be asked before debating whether or not “it” exists.
I know what I mean when I use the term; I consider it interchangeable with “volition”. Ultimately I mean that my consciousness is non-illusory; that (self-referentially) what appears to my consciousness to be my consciousness is indeed actively choosing what mental paths to pursue. What that “ME” actually is may be another question entirely, but leaving that aside for the moment, “free will” means that the sensation and experience of making choices, being actively engaged in deciding (whether for a clear reason, a murky reason, or just arbitrarily) various things is a valid, as opposed to an illusory sense.
To say that “free will” exists is to say that choices are made by an active and conscious agency. To say that “free will” does not exist is to say that no conscious agency exists, anywhere, period, end of story. In my opinion if no conscious agency exists, no consciousness exists, although if someone wishes to explain what it would mean to be conscious without agency, and to try to differentiate that state of hypothetical affairs from the complete absence of consciousness, by all means have at it.
Free will is perhaps best defined inversely as what it is not; and what it is not is radical determinism, whereby radical determinism is a belief or philosophical position that denies that anything occurs for a reason except in the sense that “reason” refers to the inertia of prior matter and energy playing itself out, sort of like a giant windup clockwork. (That had no means of “getting here” except as the outcome of some prior windup clockwork that mechanically and mindlessly produced it, so there’s a bit of a “first cause” problem with the model).
Determinism of the radical variety has arisen as a perspective in several disciplines and each has given it a slightly different emphasis —
• Sociology in its mainstream incarnation is at least semi-radical in its determinism, treating the entirety of “what makes a person” as the outcome of “enculturation”; the logical location of culture, including its values and even the terms and concepts embedded in its language, prior to the conscious individual, makes each person a blank slate (“tabula rasa”) onto which the surrounding culture writes; nothing is considered to be innately “of the person” in an individual or biological or psychological sense, but rather the entirety of who the person is is socially determined by the person’s location in time and social strata and etc.
• Biology in its essentialist incarnation is also at least semi-radical, asserting that either at the individual level (a person’s specific biology) or at the species level (the biology that we all share) every single aspect of human behavior is entirely explained including the possible library of every possible concept in every possible language, every bit of it hardwired along with all of our behaviors.
• Poststructuralist / semiotics theories of the Lacanian/Derridean/Foucauldian type are thoroughly radical determinisms, asserting even more strongly than sociology that every sliver of a conscious thought is created in a discourse of power struggle and is “always already overdetermined” by social context; that not only do individuals lack any means of transcending any of that to get at objective meanings, no objective meanings exist and there is no subject or consciousness in any real sense anyhow; only the discursive process exists, conscousness does not.
• Physical science reductionism arrives as determinism by way of seeing all of reality as physics (human thought and behavior reduced to neurochemistry which in turn reduces to atoms and molecules and charges behaving according to rules that govern them), and physics and the physical world it describes as a massive mechanically interactive process of particles in motion, with no volition anywhere, just inertia, a clockwork of some 15 billion years in age and unknown but ultimately finite size with an unknown but ultimately finite amount of energy all running down until everything disperses into a grey effuse null-state, the heat death of the universe.