A very small fish — especially the tiny ones a child catches in a stream or pond with a net and a jar. It is a British word, informal and rather affectionate, and it belongs to childhood: you would use it about the little fish in a school pond, not about anything you would eat or sell. From there it stretches easily: British people also call a very small child a tiddler, and, in business or sport, a very small company or team competing against big ones ("a tiddler taking on the giants"). In all of these the tone is warm or slightly amused, never cruel — a tiddler is small, not worthless. American English does not really use the word.