an obstruction that stands in the way (and must be removed or surmounted or circumvented)
something immaterial that stands in the way and must be circumvented or surmounted; "lack of imagination is an obstacle to one's advancement"; "the poverty of a district is an obstacle to good education"; "the filibuster was a major obstruction to the success of their plan"
small densely furred aquatic monotreme of Australia and Tasmania having a broad bill and tail and webbed feet; only species in the family Ornithorhynchidae
a military aircraft that drops bombs during flight
a large sandwich made of a long crusty roll split lengthwise and filled with meats and cheese (and tomato and onion and lettuce and condiments); different names are used in different sections of the United States
any warm-blooded vertebrate having the skin more or less covered with hair; young are born alive except for the small subclass of monotremes and nourished with milk