You won't find a "simple definition" because strand bias can mean many things, depending on the context. Basically, it can mean any process which favours one DNA strand over the other, or any physical property with a markedly-different distribution in one strand, compared with the other.

For example, a simple Google search for "dna strand bias" reveals:

• DNA Asymmetric Strand Bias Affects the Amino Acid Composition of Mitochondrial Proteins. In this case the bias is "GC skew" (a compositional difference)
• Strand bias influences the mechanism of gene editing directed by single-stranded DNA oligonucleotides. The bias here is towards one or the other strand when nucleotides are exchanged.
• DNA replication and transcription direct a DNA strand bias in the process of targeted gene repair in mammalian cells. Here, the bias refers to an increased frequency of DNA repair on one strand versus the other.

So - 3 different meanings of bias right there, depending on context.

You may consider adding more context to you question. e.g. What is your interest in the question? Are you interested in prokaryotic or eukaryotic genomes?

For prokaryotes check out this paper: DOI: 10.1007/PL00006428. It includes replication leading/lagging strand effects and gene orientation biases.