To insert anything somewhere (especially between other things), such as an affix into a word. (Compare interpolate.)
To insert an extra month into a calendar for the same purpose. The Hebrew calendar has such a month.
To insert a substance between two or more molecules, bases, cells, or tissues.
To insert an extra leap day into a calendar in order to maintain synchrony with natural phenomena.
To include (e.g. a certain word or phrase) into a sentence discreetly
To enter discreetly
To play a subtle pass into someone in a goalscoring position.