A resource allocation scheme (usually for memory). Best Fit allocates resources in a way that optimises some parameter. Alternative schemes such as first fit or random allocation are likely to be quicker but sub-optimal in use of resources.
For example, when allocating a new block of memory from a pool of free blocks (a heap), one might choose the smallest space which is big enough. This would leave larger spaces free to satisfy larger requests and reduce fragmentation of the remaining free space.