The phrase "transaction backlog" refers to a situation where there is a pile-up or delay in processing transactions. It means there are many pending or unfinished transactions waiting to be completed. Full definition
Due to the game's unbelievable popularity, the Ethereum network has been seeing record rates of transaction backlog. (bitcoinist.com)
In response, Coinbase CEO Brian Armstrong stated that «Coinbase is working on batching transactions, SegWit, and a number of other strategies to improve transaction backlog.» (cryptotimes.news)
The firm's investment banking transaction backlog increased compared with the end of the second quarter of 2011. (businessinsider.com)