a gift or act offered freely and of one's own accord, not owed or required; above all a freewill offering brought to God out of willingness rather than obligation
The word comes from a root meaning to be willing or to volunteer. At its heart it names giving that flows from a willing heart rather than from duty. What marks it out is not what is given but why: it is offered because the giver wants to, not because a law or an earlier promise compels it.
Most often it is a set term in Israel's worship, the freewill offering, a sacrifice a person could bring simply out of devotion. The law repeatedly names it alongside the vow and holds the two apart: a vow is a binding promise that must be paid, while a freewill offering is owed to no one. Because it was voluntary, its rules were a little more relaxed, and it became the natural gift of a grateful or generous heart, as when the people gave so freely for the tabernacle that Moses had to tell them to stop.
The same word also works as a description of manner, meaning freely or of one's own accord, and it stretches even to God's own giving: God promises to love his people freely, and a lavish, welcome rain can be called a rain of bounty. Across all these uses the thread is one and the same, a giving that is glad, spontaneous, and unforced.