According to WHO classification of periapical pathology, 4.81 means:
Periapical cyst
Lateral cyst
Residual cyst
Inflammatory periodontal cyst
WHO Classification:

Ko 4.4 Acute Apical Periodontitis
Ko 4.5 Chronic Apical Periodontitis
Ko 4.6 Periapical Abscess with sinus formation
	
4.60 Sinus to maxillary antrum
4.61 Sinus to nasal floor
4.62 Sinus to oral cavity
4.63 Sinus to outer skin of face


Ko 4.7 Periapical Abscess without sinus
Ko 4.8 Periapical cyst
	
4.80 Apical / lateral cyst
4.81 Residual cyst
4.82 Inflammatory paradental cyst.