Definition (from this source)
Multisig transactions have an m-of-n form, where m stands for number of signatures required to spend funds and n stands for maximum number of pubkeys that are permitted to sign (m <= n).
Alternate Definition (from this source)
Multisig transactions are also referred to as M-of-N transactions, with M being the required number of signatures or keys and N being the total number of signatures or keys involved in the transaction.