A replacement piece that is kept in a ready state as to be brought in to service without prior notice. Generally said of a "set of things".
RAID: RAID disk arrays have the capability of designating one or more of the disks in their array has a "hot spare". This disk does not contribute to the overall storage capacity of the array but instead is kept on hand with up-to-date redundancy information so that if one of the primary disks in the array fails this disk can be automatically hot swapped in to take its place.
Satellite Communications: For satellite communication systems that need high availably a number of satellites may be designated has "hot spares". In this case the satellites are in orbit and functional but are generally not being utilized. When another satellite of their group fails or or brought off-line for any reason this redundant satellite can be brought in to active duty to take over the gap in capacity or coverage left by the leaving satellite.
Hot spares in satellite communications cam pose interesting problems because, although they may be functionally equivalent to the hardware they are replacing they may or may not be locationally equivalent. As in, the spare is in a different position, however slight, to the original and thus small edge cases can appear with regard to reception at stations where te signal was already marginal.
In the same way that there are "hot spares" there are also "cold spares", which is to say a replacement part that is not immediately ready for service, although this term is generally taken as just "spare" with no designator. Sometimes this is humorously used to refer to parts that are not on site and must be ordered from a supplier.
Still other companies in technology have used terms like "warm spare" to denote spares that, although not completely ready for swap-in at a given moment, require less time to be brought in to service than a "cold spare".
The term "hot spare" can sometimes be used humorously of people in that there may be two people in an organization with overlapping skills and may, for the sake of comedy, be said to be interchangeable.