The phrase "drop in" can be defined as visiting someone suddenly and without prior notice. There are a variety of synonyms for this phrase that can be used to convey a similar meaning. These include "pop in," "stop by," "pay a visit," "swing by," "call by," and "fall in." Each of these phrases invites someone to arrive spontaneously and without an appointment or formal invitation. They can be used in any social context where someone wants to see someone else and don't want to arrange a meeting in advance. Whether popping in on a friend at their house or stopping by their office, these phrases are useful and flexible ways to communicate a visit.