For the purpose of this subchapter, the term “encumbrance” shall mean a commitment for expenditure of an appropriation evidenced by a valid purchase order, similar document or process for the acquisition of supplies, material, work or services.

9 Del. C. 1953, § 1137; 55 Del. Laws, c. 85, § 1; 71 Del. Laws, c. 401, § 36; 83 Del. Laws, c. 7, § 4