“a’” (a with an apostrophe) is not a standard standalone English word, but it can appear in a few contexts:

• Shorthand / stylistic writing: sometimes used informally to represent a variant or modified “a”, or as a placeholder in notes and transcripts.
• Mathematics / science notation: it can mean “a prime” (a′), which usually represents a modified version of a variable “a” (e.g., a transformed or derived value).
• Typo or fragment: often just “a” with an accidental apostrophe.

Most commonly in technical contexts, a′ = “a prime” = a related or changed version of a.