1. A branching tubular cell characteristic of the filamentous fungi (molds). In most species the hyphae are divided by cross-walls (septa) into multicellular hyphae; intercommunicating hyphae constitute a mycelium, the visible colony on natural substrates or artificial laboratory media. The terms hypha and mycelium often are used interchangeably. 2. Similar structure in some bacteria, e.g., Streptomyces.