Hesitation marker

A hesitation marker is a linguistic form that appears in environments in which speakers have difficulties in retrieving lexical information during speech production.

Examples
English whatchamacallit, thingamy, German Dingens

Reference

 * Hayashi, Makoto & Yoon, Kyung-eun. 2006. A cross-linguistic exploration of demonstratives in interaction. Studies in Language 30:485-540.