From Longman Dictionary of Contemporary Englishbe boardingbe boardingTTif a plane or ship is boarding, passengers are getting onto it Olympic Airways Flight 172 to Istanbul is now boarding at Gate No. 37. → board
Examples from the Corpus
be boarding• People were boarding and I elbowed my way into line.• They want a meeting, they intimate, and are boarding for that purpose now.• There was more to this, when Sammler was boarding in the tomb.• The other option to consider will be boarding kennels. 